50 POPULAR CONCHOLOGY. 



on each side the body. — 16 recent and 98 fossil 

 species. 



These animals inhabit great depths, and fix 

 themselves to other objects by filaments or a 

 tendinous cord which proceeds from the hole 

 in the boss. They have a curious kind of in- 

 ternal skeleton, as it may be termed, consisting 

 of a slender, flattened, calcareous loop, with 

 other pieces diverging from it : the arms of the 

 animal seem to be attached to it. They are 

 found in the European seas, those of N. Zea- 

 land, and the Southern Ocean. 



Family 2. — Adhcerentia* 



This family attach themselves by the shell ; 

 there is still, however, a muscular tendon in most 

 of them, passing through a perforation in the 

 lower valve. The valves are not united by any 

 ligament. The shell is thin, and rather horny 

 than shelly. — There are 3 genera. 



3. Thecidium (Defrance). — Shell ovate, 

 small, inequivalve, nearly equilateral, imperfo- 

 rate, and attached by the concave valve, which 



* From adhcereo, Lat., to adhere. 



