8 - 'it Hi-' fti-I'i'ii / /'/v.v//Kv/Ar F'umat. 105 



for we can easily (inil close alliance.- for tlicin amongst marine molluscan families 

 from which we may regard them as directly or collaterally defended. Further, 

 as fossil remains (l f marine molluscs are comparatively abundant, we shall be able 

 from the first appearance of these to gather some idea as to the probable date of the 

 first appearance of their freshwater relations; always, however, bearing in mind that 

 the determinations <,f many I'al;eo/.<>ic Mollusca are to a certain extent doubtful. 

 St/tt ing these aside, we find that the Cyprinidre (in sensu restricto) date from the Trias, 

 and thus ( 'vrena, which is allied to this family, in all probability may be regarded 

 , ost-Trias.Mc in age. The Neritina; are included in the family Neritidse, which 

 is also lirsi found in the Trias, while Hydrobia belongs to the Rissoidae, first mot 

 with in the Juni, though it may of course have made its appearance earlier, and 

 proKalilv did. Thus the genera Cyrena, Neritina, and Hydrobia cannot safely be 

 pushed farther back than the Trias, but may very possibly have then originated 

 in the lakes of that period. 



Passing now to the Purbeck-Wealdeu deposits, we encounter the numerous 

 'iwater genera detailed in the following list: Unio, Cyrena, Corbula, Cardium, 

 Valvata, Hydrobia. Amnicola, Xeritina, Plauorbis, Lioplax, Bithynia, Paludina (?), 

 Physa, LimiKeus, (hiathodon, Pleuroceras, Goniobasis, Leptoxis, Plychostylus, 

 ( 'arvchiuni. and Auricida. Of the genera which here appear for the first time, Gna- 

 thodon, a sub-genus of Mactra, may have become specially modified in Jurassic 

 times, for the .:< mis is not known earlier than the Coral-rag. Unio, however, may 

 be regarded as a descendant of the Devonian Anodonta. The list is more espe- 

 cially i: ig as affording evidence of the abundant development of the fresh- 

 water MelauiiiKc in our own area during the Mesozoic period, from which, as well as 

 from Xorth< in Europe generally, they arc now entirely absent, though widely spread 

 in other regions, particularly the warmer zones of the earth. Considering the small 

 part of the earth's crust that has been at all carefully studied, it is of the highest 

 interest to find the same genera of freshwater shells occurring at widely-separated 

 points in bed- of approximately contemporaneous age, and so far remote from the 

 present as ("'retacemis times (Cenomanian or Senonian). Thus in North America, 

 where the Cretaceous rocks have been grouped as follows: Laramie, Fox Hill, 

 Colorado, and Dakota groups, they have afforded Unio (one species), Cyrena (one 

 >pccies), Xeritina (three species), Physa, and Valvata, from the Fox Hill bed-: 

 and Unio, Corbicula, Acella, Leptolimnea, Limnophysa, Limnsea, Neritina, Mela- 

 nopsis, ( 'ampelosis, Pyrgulifera, and others, from the Laramie beds. With regard 

 to the fauna of the Fox Hill beds, Clarence King well remarks that " the disco- 

 very of this singularly tertiary-like group deep in the Cretaceous should only 

 open our eyes to the early specialisation of freshwater molluscan types." And 

 Dr. White, referring to the Laramie fossils and their similarity to existing forms, 

 speaks more strongly still, as when he says : " The lines of descent of the nume- 

 rous types which have reached us unbroken seem to be almost parallel, so little 



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