This research is based on different sources, many of which are available online [1]. However, several of them need some digging to get all the data out; an important reason to sort out the snail pictures and complementary documentation and present them here in this blog. Round-ups, analyses and reflections may be done in due time.

As depositories I have used the following so far [2]; additional sources may be added later by an update of this post.
AIC – Art Institute of Chicago, Chicago, U.S.A.
AMV – Albertina Museum, Vienna, Austria
APM – Alte Pinakothek, Munich, Germany
AVH – Art Gallery Johnny Van Haeften, London, U.K.
BBR – Museum Boijmans van Beuningen, Rotterdam, the Netherlands
BIM – Bibliothèque Interuniversitaire, Montpellier, France
BLL – British Library, London, U.K.
BLY – Bibliothèque municipale, Lyon, France
BMB – Bowes Museum, Barnard Castle, Teesdale, U.K.
BML – British Museum, London, U.K.
BMM – Bash Museum, Miami, U.S.A.
BMP – Bibliothèque Mazarine, Paris, France
BLO – Bodleian Library, Oxford, U.K.
BNF – Bibliotheque National, Paris, France
BRB – Bibliothèque Royale, Brussels, Belgium
BSB – Bayerische StaatsBibliothek, Munich, Germany
CEM – Musée des Civilisations de l’Europe et de la Méditerranée, Marseille, France
CMA – Currier Museum of Art, Manchester, NH, U.S.A.
CMS – Chateau-Musée Saumur, Saumur, France
CUN – Columbia University, Rare Book and Manuscript Library, New York, U.S.A.
DKC – Det Konigle Bibliothek, Copenhagen, Denmark
DKU – Domkerk, Utrecht, the Netherlands
FAH – Museum of Fine Arts, Houston, TX, U.S.A.
FBN – Musée Frédéric Blandin, Nevers, France
FLP – Free Library of Philadelphia, Philadelphia, U.S.A.
FWC – FitzWilliam Museum, Cambridge, U.K.
GAK – Gemäldegalerie Alte Meister, Kassel, Germany
GJC – Musée Gallé-Juillet, Creil, France
GPF – Galleria Palatina (Palazzo Pitti), Florence, Italy
GUF – Galleria degli Uffizi, Florence, Italy
HAB – Herzog August Bibliothek, Wolfenbüttel, Germany
HAM – Harvard Art Museums, Cambridge, MA, U.S.A.
HMB – Herzog Anton Ulrich Museum, Braunschweig, Germany
JMA – Johnson Museum of Art, Cornell University, Ithaca, U.S.A.
KBH – Koninklijke Bibliotheek, The Hague, Netherlands
KMW – Kunsthistorisches Museum, Vienna, Austria
KTP – Kharbine Tapabor, Paris, France
LAM – St. Louis Art Museum, St. Louis, U.S.A.
LLB – Lilly Library, Indiana University, Bloomington, U.S.A.
MAB – Musée des Beaux-Arts et d’Archeologie, Besançon, France
MAR – Memorial Art Gallery, Rochester, U.S.A.
MBB – Musée des Beaux-Arts, Bordeaux, France
MBC – Musée des Beaux-Arts, Chambéry, France
MBD – Musée des Beaux-Arts, Dijon, France
MBG – Museo de Bellas Artes, Granada, Spain
MBN – Musée des Beaux-Arts, Nancy, France
MBO – Musée du Berry, Bourges, France
MBR – Musée des Beaux-Arts, Rennes, France
MBV – Musée des Beaux-Arts, Valence, France
MCP – Musée nationale de Chateau de Pau, Pau, France
MHH – Mauritshuis, The Hague, Netherlands
MIA – Minneapolis Institute of Arts, Minneapolis, U.S.A.
MJD – Musée Joseph Déchelette, Roanne, France
MLP – Musée du Louvre, Paris, France
MMA – Metropolitan Museum of Art, New York, U.S.A.
MNG – Magyar Nemzeti Galéria, Budapest, Hungary
MSK – Koninklijke Musea voor Schone Kunsten, Brussels, Belgium
MTH – Musée Thomas Henry, Cherbourg, France
MUM – Musée des Ursulines, Mâcon, France
ÖNB – Österreichische Nationalbibliothek, Vienna, Austria
NGI – National Gallery, Dublin, Ireland
NGL – National Gallery, London, U.K.
NMZ – Nationalmuseum, Zurich, Switzerland
PAM – Phoenix Art Museum, Phoenix, U.S.A.
PBL – Palais des Beaux-Arts, Lille, France
PDV – Palazzo Ducale, Venice, Italy
PGM – Paul Getty Museum, Malibu, U.S.A.
PMA – Philadelphia Museum of Art, Philadelphia, U.S.A.
PMM – Pierpont Morgan Museum, New York, U.S.A.
RAM – Royal Albert Memorial Museum & Art Gallery, Exeter, U.K.
RGS – Residenzgalerie, Salzburg, Austria
RKD – Rijksbureau voor Kunsthistorische Documentatie, The Hague, Netherlands
RKU – Ruprecht-Karls Universität, Heidelberg, Germany
RLB – Rheinisches Landesmuseum, Bonn, Germany
RMA – Rijksmuseum, Amsterdam, Netherlands
RMT – Rijksmuseum Twenthe, Enschedé, Netherlands
SKD – Staatliche Kunstsammlungen, Dresden, Germany
SKK – Staatliche Kunsthalle, Karlsruhe, Germany
SMA – Stedelijk Museum, Amsterdam, Netherlands
SMK – Statens Museum for Kunst, Copenhagen, Denmark
SSO – Musée de l’hotel Sandelin, Saint Omer, France
TCC – Trinity College, Cambridge, U.K.
TLI – Tiroler Landesmuseum, Innsbruck, Austria
TMH – Teylers Museum, Haarlem, Netherlands
TWC – The Wilson Cheltenham Art Gallery and Museum, Cheltenham, U.K.
ULL – Université de Liège, Liège, Belgium
UPC – Unknown Private Collection
UTL – University of Toronto Libraries, Toronto, Canada
VAG – Vancouver Art Gallery, Vancouver, Canada
VAM – Victoria and Albert Museum, London, U.K.
VUA – Vrije Universiteit, Amsterdam, Netherlands
WAB – Walters Art Museum, Baltimore, U.S.A.
WLL – Wellcome Library, London, U.K.
YBA – Yale Center of British Art, New Haven, U.S.A.

A glossary of terms used is below, also updated when new terms are added:
border – margin (partially) around or between text, fully illustrated with animals, plants, objects, and human or imagined figures.
dextral – see ‘right-coiled’.
epiphragm – mucous cover of the aperture, used by the snail to prevent dehydration during prolonged resting periods.
left-coiled – with the apex pointing up, the aperture opens to the left; this is indicated in malacology as ‘sinistral’.
marginalia – figures around a text, freestanding or loosely coupled to the text with lines.
miniature – framed figure showing representations from (imagined) life.
right-coiled – with the aperture pointing up the aperture of the shell opens to the right; in malacology this is called ‘dextral’.
sinistral – see ‘left-coiled’.
‘2D squashed’ – the 3D shape of a snail transferred to a 2D plane, usually with the body in side view and the shell in dorsal view (i.e. from the top); in reality this top is at some angle to the side view plane, depending on the shape of the shell (ranging from elongate or globose to discoid).

The following references have been very useful to get an understanding of the context in which I found the snails:
Allmon, W.D. (2007). The evolution of accuracy in natural history illustration: reversal of printed illustrations of snails and crabs in pre-Linnean works suggests indifference to morphological detail. Archives of natural history 34: 174–191.
Kuechen, U.-B. (1979). Wechselbeziehungen zwischen allegorischer Naturdeutung und der naturkundlichen Kenntnis von Muschel, Schnecke und Nautilus. In: W. Haug. (ed.) Formen und Funktionen der Allegorie: 478–514. J.B. Metzlersche Verlagsbuchhandlung, Stuttgart.
Pinon, R. (1980). From illumination to folksong; the armed snail, a motif of topsy-turvy land. In: Newall, V.J. (ed.), Folklore studies in the twentieth century. Proceedings of the Centenary Conference of the Folklore Society: 76–113. Boydel & Brewer, Woolbridge / Rowman & Littlefield, Totowa.
Randall, L.C. (1962). The snail in Gothic marginal warfare. Speculum 37: 358–367.

David Robinson (Philadelphia) originally sent me a link to a blogpost on the medieval theme of ‘knight versus snail’. That greatly provoked my interest and is thankfully acknowledged here. The information of and discussion with Marrigje Rikken (Leiden) was stimulating and helpful.

[1] Some meta-sources that have proven to be very helpful while hunting for snails:
– Database of the Netherlands Institute for Art History (explore.rkd.nl);
– Website of Siân Echard, with many links to further sources: Medieval Manuscripts on the Web (http://faculty.arts.ubc.ca/sechard/512digms.htm);
– Joconde: database of the collections of the state museums of France (http://www.culture.gouv.fr/public/mistral/joconde_fr).
[2] Important disclaimer: All efforts have been made to present the snail pictures in their original setting. The original photographs are referred to in corresponding notes under each post; the copyrights for the context picture(s) are stated there and implicitly apply also to my blog posts, unless further stated. Users of pictures from this blog therefore should consult the links mentioned in the respective notes. Detailed enlargements have been made using these original photographs, unless otherwise mentioned. If readers have contributed pictures for this blog, they have granted publication permission in email correspondence; they are acknowledged in notes with each applicable blogpost. Otherwise photos are my own.