1 8 A SYNOPSIS OP ANIMAL CL/ISSIFICATION. 



( ' ' Lamellibranch iata " ) . The great 

 majority of the Pelecypoda are 

 here included. x Mya, Venus. 



Order 5. Septibranchia Ctenidia reduced to muscular par- 

 titions perforated by rows of gill 

 slits and dividing the mantle cavity 

 into two compartments. 



X Silenia. 



The following somewhat older classification of the Pelecypoda was 

 based upon the presence or absence of a siphon and the conditions of 

 the adductor muscles, a system which had the advantage of being appli- 

 cable to the shells from which the soft parts have been removed, but 

 which is not so fundamental morphologically as the present one. 



Order i. Siphoniata With two posterior siphons, separate 



or fused. Edges of mantle often 

 joined. 



Sub-order I. Sinupalliata Siphons long and contractile. Pal- 



lial line with sinus. 

 X Mya, Venus, Solen. 

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



pallial sinus, x Cyclas. 

 Order 2. Asiphonia Siphons absent. 



Sub-order I. Homomyaria Anterior and posterior adductor 



muscles about equal, x Unto. 



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



X Mytilus. 



Sub-order 3. Monomyaria Anterior adductor wanting. 



X Ostraa, Pecten. 



Class III. SCAPHOPODA Bilaterally symmetrical molluscs 



with visceral sac greatly elongated 

 in a dorso-ventral direction. 

 Mantle and shell tubular and 

 somewhat curved, with a smaller 

 dorsal and a larger ventral open- 

 ing. Ctenidia fail. Foot elon- 

 gated and conical. A single family 

 of marine forms. x Denialium. 



Class IV. GASTEROPODA Molluscs with a head, foot, and 



visceral sac. The first two are 

 bilateral; the third is generally 

 contained in a spirally twisted 



