//. TYPES IN DETML IVITH THEIR SUBORDIN/ITE GROUPS, i? 



are without a distinct head and 

 lack a "lingual ribbon" or ra- 

 dula. The foot is compressed and 

 never forms a creeping disc. 



Order I. Protobranchia With normal, doubly pinnate 



ctenidia extending backwards in 

 the mantle cavity, and free, ex- 

 cept at the base. 



X Nucula, Yoldia. 



Order 2. Filibranchia. Separate plates of the ctenidia 



filiform and greatly lengthened, 

 and folded back upon themselves. 

 The separate filaments are free from 

 one another, but are so closely 

 approximated that they give the 

 appearance of continuous plates. 

 X Mytilus, Area. 



Order 3. Pseudolamellibran- 



chia Similar to the last, but with slight 



adhesions between the separate 

 filaments, caused by ciliated sur- 

 faces; also with a few cross-bars, 

 with vascular connexion between 

 the descending and ascending limbs 

 of each filament and with a vertical 

 folding or crimping of the lamellae 

 as a whole, forming ridges and de- 

 pressions. X Pecien, Osircea. 



Order 4. Eulamellibranchia... Filaments so completely fused as 



to be no longer recognizable as 

 such, and with the descending and 

 ascending lamellae thus formed so 

 frequently united by their adjacent 

 surfaces that the entire ctenidium 

 of one side appears in the form 

 of two gill-plates, a characteristic 

 once used as typical of the class 



