MANUAL OF NATURAL HISTORY. 151 



II. ORDEK.— Cladopods (Cladopoda). 

 F(K)t large and club-shaped, often truncate and 

 expanded at the end ; mantle-lobes usually united, 

 with a passage in front for the foot ; siphons large, 

 produced, generally united to their ends ; gills pro- 

 duced into the anal siphon. 



1. Family. — Borers (Pholadidae). Siphons greatly 



prolonged, united as far as their ends ; foot 

 club-shaped, truncate at end ; shell free, or 

 within a tube, without ligament, and with 

 apophyses under the beaks ; often with sup- 

 plemental valves. 



2. Family. — Tube-Shells (Gastrochsenidse). Siphons 



very long, united almost to their ends and 

 fimbriated ; foot small, digitiform ; shell equi- 

 valve, often gaping, incrusted in whole or 

 partly, or free in a tube, which is either free 

 or enclosed ; no apophyses at the hinge. 



3. Family. — Razor-fish (Solenidse). Siphons short, 



united ; orifices fimbriated ; foot elongated, 

 thick, club-shaped, truncated ; shell sub- 

 cylindrical, greatly elongated transversely, 

 gaping at each end ; hinge with two or three 

 teeth in each valve, the hinder bifid, carti- 

 lage external, on a pad or fulcrum. 



4. Family. — Pod-Shells (Pharidse). Siphons sepa- 



rated for more than half their length ; foot 

 ovate, elongate, truncated ; shell greatly 

 elongated transversely, gaping at each end • 

 ligament external ; hinge simple, or with 

 one or two hooked teeth. 



