40 INVERTEBRATE ZOOLOGY. 



Order 1. Solenogastres Worm -like forms without mantle or 



shell, but with calcareous spicules im- 

 bedded in the cuticula. Some possess a 

 ciliated ventral furrow, in the bottom of 

 which lies a rudimentarj' foot. In others 

 furrow and foot fail. -^ Proneomenia. 



Order 2. Chitones Depressed forms with a dorsal shell com- 

 posed of eight transverse pieces. Cteni- 

 dia in two longitudinal rows between 

 foot and edge of mantle. Foot large, 

 with oval, flattened creeping surface. 

 ■^ Chiton. 



Class II. Lamellibranchiata Bilaterally symmetrical Mollusca with 



four (in a few cases two) plate-like gills, 

 and with two lateral shells generally 

 united dorsally. They are without a dis- 

 tinct head and lack a ' ' lingual ribbon " 

 or radula. The foot is compressed and 

 never fonns a creeping disc. 



Order 1. Siphoniata With two posterior siphons, separate or 



fused. Edges of mantle often joined. 



Siib-order 1. SinupaUiata Siphons long and contractile. Mantle 



line with sinus. -^ Mya, Venus, Solen. 



Sub-order 2. Integripalliata .... Siphons short, not contractile. No pal- 



lial siniis. ^ Cyclas. 



Order 2. Asiphonia Siphons absent. 



Sub-order 1. Homontyaria Anterior and posterior adductor muscles 



about equal, if Unto. 



Sub-order 2. Heteromyaria . . . .Anterior adductor very small. 



ir Mytilus. 



Sub-order 3. Monomyaria Anterior adductor wanting. 



if Ostrea, Pecten. 



