molluscoida: distribution. 297 



Fam. VII. Prodactida. 



Animal unknown. Shell entirely free, or attached to rharine bottoms 

 by the substance of the beak ; valves either regularly articulated, or 

 kept in place by muscular action. No calcified support for the oral 

 appendages. 



Genera. — Prodttcta, Ckottetes, Strophalosia, Auhsteges. 

 Fam. VIII. Craniadee. 



Animal fixed to submarine objects by the substance of the shell of 

 the ventral valve. Arms fleshy and spirally coiled ; no hinge or 

 articulating processes; upper or dorsal valve patelliform {}.e. limpet- 

 shaped). 



Genus. — Crania. 

 Fam. IX. DiscinieUe. 



Animal attached by means of a muscular peduncle passing through 

 the ventral or lower valve by means of a slit in its hinder portion, or 

 a circular foramen excavated in its substance. Arms fleshy, valves 

 unarticulated. 



Genera. — Discina, Trematis, Sipkonotreta, Acrotrela. 

 Fam. X. Lingulidce.* 



Animal fixed by a muscular peduncle passing out between the beaks 

 of the valves ; arms fleshy, unsupported by calcified processes. Shell 

 unarticulated, sub-equivalve, texture homy. 

 Genera. — Lingiila, Obolus. 



CHAPTER XLV. 



DISTRIBUTION OF MOLLUSCOIDA. 



DibTRiBUTiON OF MoLLUSCOiDA IN SpACE. — The Polyzoa, like 

 all the Molluscoida, are exclusively aquatic in their habits, but, 

 unlike the remaining two classes, they are not exclusively con- 

 fined to the sea. The marine Polyzoa are of almost universal 

 occurrence in all seas. The fresh-water Polyzoa, however, not 

 only differ materially from their marine brethren in structure, 

 but appear to have a much more limited range, being, as far as 

 is yet known, confined to the north temperate zone. Britain 

 can claim the great majority of the described species of fresh- 

 water Polyzoa, but this is probably due to the more careful 

 scrutiny to which this country has been subjected. 



The Tunicata are cosmopolitan in their distribution, and are 

 foiuid in all seas, the Mediterranean appearing to be especially 

 rich in members of this class. Four genera are pelagic in their 

 habits, and several are found in the Arctic regions. 



* Another <amily was fonnerly constituted for the reception of the sin- 

 gular Devoniin fossils known as Calceola. It has been shown, however, 

 that these are probably operculate. Corals. 



