502 MOLLUSCA phylum vi 



are unknown from strata geologically older than the St. Peter sandstone, whicli is of 

 early Ordovician age. Here appear suddenly several genera of the Modiolopsidae, and 

 this family, together with the Anibonychiidae, Cyrtodontidae, and Ctenodontidae, 

 attain the acme of their development during the Middle and Upper Ordovician. In 

 the Silurian a considerable number of bivalves is observable, as many as eighty 

 species having been distinguished in the fauna of the small island of Gotland alone. 



A very marked difference in geological ränge is perceptible among the three Orders 

 into which the class is divided. The Prionodesmacea, including most of Neumayr's 

 Palaeoconcha, are pre-eminently characteristic of the Paleozoic faunas. Of the forty- 

 two families referred to this order, no less than seven occur in the Ordovician, and 

 eighteen in the Silurian, to which seven are added during the Devonian, only three in 

 the Carboniferous, and one in the Permian. From these ancient Stocks only seven 

 Prionodesmacean families are evolved during the whole of the Mesozoic, and but two 

 in the Tertiary, while three are Eecent. 



The Order Änomalodesmacea is represented in the Paleozoic solely by its radical, 

 the Pholadellidae ; eight of its sixteen families originate in the Mesozoic and Tertiary, 

 and, with the exception of the Pholadellidae and Pleuromyacidae, all have endured 

 until the present time. Only one family appears to be exclusively Eecent. 



The Teleodesmacea are distinctively modern, although foreshadowed in the Paleo- 

 zoic by Cypricardian, Lucinoid and Allodesmid radicals (the Solenoid radical is still 

 questionable). Of forty -seven families thirty can be first definitely recognised in the 

 Mesozoic, twelve originate in the Tertiary, two are exclusively recent, and only a single 

 one can be traced continuously from the Paleozoic to the recent fauna. 



Of the Prionodesmacean families, lO'ö per cent survive ; of the Teleodesmacean 71 

 per cent ; and of the Anomalodesmacean 88 per cent. If it were not for the mortality 

 among the Chamacea and Rudistacae, the ratio of survival among the Teleodesmacean 

 families would be 95 per cent. Of 105 families which have been discriminated during 

 the whole history of the class, 76, or about 72*3 per cent, are represented in the 

 existing fauna. Families have originated in the various geological epochs as follows : 

 Ordovician 9, Silurian 11, Devonian 9, Carboniferous 3, Permian 1, Trias 13, Jura 14, 

 Cretaceous 18, Eocene 15, Miocene and Pliocene 3, Pleistocene and Recent 6. From 

 this it appears that the development of the group, judged by the increase of families, 

 was most intense during the Silurian, thereafter rapidly decreasing until the Trias, 

 then gradually increasing until the Cretaceous, after which the rate of differentiation 

 again rapidly declined. It is noted that in the Paleozoic the Pelecypods form about 

 one-quarter of all the moUusks known from this era ; in the Jura and Cretaceous 

 about one-half, and in the Tertiary about one-third of this number. 



The Ordovician and Silurian are especially characterised by the presence of 

 Taxodont, Palaeoconch, and the older forms of Schizodont Pelecypods. The Cardio- 

 lidae, Pterineidae, Ambonychiidae and Modiolopsidae are common to both the Silurian 

 and the Devonian. 



The Devonian has no families solely characteristic, but the brackish-water Car- 

 diniidae, the Megalodontidae, Trigoniidae, Pinnidae, Pectinidae and Mytilidae first 

 take rise in this period, and the sinupalliate Ällorisma is the first Pelecypod showing 

 clear evidence of retractile siphons. 



The Carboniferous is marked by the appearance of Parallelodon and its allies, 

 the Limidae and Ostreidae, and some precursors of the Lucinacea and Pholadacea. 

 The Pernidae and Gastrochaenidae make their advent in the Permian ; but, on the 

 whole, the Carboniferous fauna persists throughout this period. In the Trias, however, 

 important changes take place ; many old genera disappear, and such forms as the Limop- 

 sidae, the true Uniones, Spondylus, Dimya, the Pleuromyacidae, Pholadomyacidae, 

 Astartidae, Lucinacea, Cardiidae and Corbulidae enter upon the scene. 



During the Jura, genuine Arcidae, Änomia, Eligmus, various Anatinacea, Cyrena^ 

 Diceras, Isocardia, and the Teleodont Veneridae, Tellinidae, Donacidae and Pholadacea 



