CLASSIFICA TION OF BIVAL VES. 3 1 3 



Sub-order 2. Sinupallia. The mantle's line of attachment to the 

 shell is inflected by a sinus into which the 

 large siphons are retracted. 

 Venus, Mya, Saxicava (a boring bivalve), Solen (razor- 

 shell), Pholas (borer), Teredo (ship-worm), Asper- 

 gillum (watering-pot shell). 



Order 2. Heteromya. The anterior adductor is much smaller than 

 posterior, and siphons are rare. 



Mytiliis (edible mussel), Modiola (horse-mussel), Litho- 

 donius (borer), Dreissena. 



Order 3. JMoNOHYA. One adductor, no siphon. 



Ostrea (oyster), Anomia, Lima, Pecten (scallop). 



Class Gasteropoda. Snails and slugs of many kinds, some 

 living on land, others in water. 



This class includes not only the common air-breathing 

 snails and slugs, but the marine whelks and periwinkles, 

 sea-hares (Aplysia) and " Nudibranchs " like Doris, limpets 

 and ear-shells (Haliotis). To form a conception of Gastero- 

 pods from the snail alone, is apt to be very misleading. 



In fact, there are so many different kinds of Gasteropods, 

 that some division of the class is necessary before we can 

 formulate general characters of much usefulness. Con- 

 trasted with bivalves, Gasteropods have usually a single shell, 

 the head region is more or less distinctly developed, the 

 mouth contains a rasping ribbon, and the foot, except in 

 those adapted for free swimming, is a flat median sole on 

 which the animal creeps or rests. In the foot distinct fore, 

 mid, and hind regions are often well marked. 



General Survey. 



A. Symmetrical Gasteropods (Isopkura.) — The Gasteropods which seem 

 most primitive are forms like Chiton, some species of which, easily 

 known by their eight shell-plates, are not uncommon on the shore rocks. 

 They are symmetrical, not lop-sided like most of the higher forms. 

 The food-canal runs from one end of the body to the other ; the gills, 

 kidneys, auricles, and genital ducts are paired ; two pairs of nerve- 

 cords (pedal and visceral) run parallel to one another along the body, 

 and the ganglia are but slightly developed. Agreeing in essential 



