This painting of Carlo Crivelli (c. 1435–c. 1495) is in the National Gallery, London and there entitled “The Virgin and Child with Saints Francis and Sebastian” . Crivelli was an Italian Renaissance painter, who was born in Veneto but later moved to the Marche of Ancona.
Unlike the naturalistic trends arising from Florence at the same time, Crivelli’s style still echoes the courtly International Gothic sensibility. The urban settings are jewel-like, and full of elaborate allegorical detail. 
The work is described as “Oradea, the widow of Giovanni Becchetti, founded an altar dedicated to S. Maria della Consolazione (Holy Mary of Consolation) in the church of San Francesco at Fabriano in 1490. This fulfilled her husband’s wish, expressed in his will.
Crivelli painted the altarpiece in the following year. The inscription along the base in Latin emphasises the expense of the commission. The inclusion of Oradea’s portrait reflects the widow’s pride in her commission from the leading artist of the Marches, who had been knighted. He refers to his title in his signature. Saint Francis appears because the church was of the Franciscan order. Saint Sebastian was a knight and martyr. Many of the objects in the picture are symbols of Mary as the Virgin Mother of Christ. The transparent glass is a symbol of Mary’s virginity. Fruit can be a sign of fertility. Christ is referred to as the fruit of Mary’s womb. The white lily is Mary’s special flower, symbolising purity. Snails were (erroneously) thought to reproduce asexually, referring to the Immaculate Conception. Among the flowers there is a carnation, sometimes a symbol of Christ’s Passion.” 
In the lower left corner on the floor, a naturalistically painted snail may be seen, likely a local, Umbrian Helix species. This is likely one of the first (if not the first) shelled snails on a painting that is drawn after nature.
 NGL, NG807, dated 1491, egg and oil on poplar. http://bit.ly/1qFrN6i (15.iii.2014).