44 COOKE : ORIGIN OF THE GENERA OF L. AND F. W. MOLLUSCA. 



from Cardium (Adacna, Didacna, Monodacna), have already been 

 referred to. Nausitora is a form of Teredo, which lives in fresh 

 water in Bengal. Rangia, Fischeria, and Galatea probably share 

 the derivation of the Cyrenidce, while in Iphigenia we have one of 

 the DonaciJic which has not yet mounted rivers, but is confined to a 

 strictly estuarine life. The familiar Scrobicularia piperata of our own 

 estuaries is a Tellina, which lives by preference in brackish water. 



The great family of the Unionidce is regarded by Neumayr* as 

 derived from Trigonia, the points of similarity being the development 

 of a nacreous shell, the presence of a strong epidermis, and the 

 arrangement of the muscular scars. It is remarkable, too, that on 

 many Uniones of Pliocene times there is found shell ornamentation 

 of such a type as occurs only on Trigonia among the Pelecypoda. 



The earliest types of freshwater Pelecypoda occur in the 

 cretaceous ( Unio, Cyrena). 



The genera of freshwater Pelecypoda are comparatively few in 

 number, and their origin is far more clearly discernible than that of 

 any other group. This is perhaps due to the fact that the essential 

 changes of structure required to convert a marine into a freshwater 

 bivalve are but slight. Both animals " breathe water," and both 

 obtain their nutriment from matter contained in water. Similar 

 remarks apply to freshwater operculate Gasteropoda. But the 

 passage from a marine to an aerial life involves much profounder 

 changes of environment, which have to be met by correspondingly 

 important changes in the organism. This may be in part the reason 

 why the ancestry of all Pulmonata, whether land or freshwater, is so 

 difficult to trace. 



(is) GASTEROPODA, (i) Operculate. 



Canidia, Clea, and perhaps Nassodonta are forms closely allied, 

 with but little modification, to the marine Cominella.\ They occur 

 (in fresh water) in the rivers of India, Indo-China, Java, and Borneo, 

 associated with essentially freshwater species. Potamides with its 

 various subgenera ( Telescopium, Pyrazus, Jirenella, Cerit/iidea, &c), 

 all of which inhabit swamps and mudflats just above high-water mark 

 in all warm countries, are derived from Cerithium ; Assiminea, 

 Hydrobia, and perhaps Truncatella, from Rissoa. It is a remarkable 

 fact that in Gcomelania (with its sub-genera Chittya and Blandielia) 

 we have a form of Truncatella which has entirely deserted the 

 neighbourhood of the sea, and lives in woody mountainous 

 localities in certain of the West Indies. Cremnocotuhus, a remark- 



* Anz. K. Akad. Wiss. Wien, 1889, p. 4. 



t Not to Nassa, as has been generally held. The shape of the operculum, and particularly 

 the teeth of the radula, show a much closer connection with Coniinella. 



