ORDER III RANGE OF THE PELECYPODA 425 
the Silurian, and of these the recent representatives are everywhere recognised as 
constituting the most distinctively archaic type of Pelecypod structure now existing. 
The order Anomalodesmacea is represented in the Palaeozoic 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 Recent. 
The Teleodesmacea ave distinctively modern, although foreshadowed in the Palaeo- 
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 Palaeozoic to the recent fauna. 
Of the Prionodesmacean families, 10-5 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 Rudistae, 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 :— 
Cambrian 1, Silurian (Murchisonian) 20, Devonian 10, Carboniferous 3, Permian 2, 
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 Palaeozoic 
the Pelecypods form about one-quarter of all the mollusks known from this era ; in the 
Jura and Cretaceous about one-half, and in the Tertiary about one-third of this iii 
The Silurian is especially characterised by the presence of Taxodont, Palaeoconch, and 
the older formsof Schizodont Pelecypods. The Vlastidae, Cardiolidae, Antipleuridae, Cteno- 
dontidae, Pterineidae, Ambonychiidae, and Modiolopsidae are shared with the Devonian. 
The Devonian has no families solely characteristic, but the brackish water Car- 
diniidae, the Megalodontidae, Trigontidae, Pinnidae, Pectinidae, and Mytilidae first 
take rise in this period, and the sinupalliate Allorisma 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 Ostreidac, and some precursors of the Lueinacea and Pholadacea. 
The Pernidae and Gastrochaenidae make their advent in the Permian; but, on the 
whole, the Carbonic fauna persists throughout this period. In the Trias, however, 
important changes take place; many old genera disappear, and such forms as Limop- 
sidae, the true Uniones, Spondylus, Dangen the Plewromyacidae, Lites Tae aeCees 
Astartidae, Lucinacea, Cardiidae, and Corbulidae enter upon the scene. 
During the Jura, genuine Arcidae, Anomia, Eligmus, various Anatinacea, Cyrena, 
Diceras, Isocardia, and the Teleodont Veneridae, Tellinidae, Donacidae, and Pholadacea 
are initiated. The character of the Cretaceous is strongly influenced by the aberrant 
and short-lived Chamacea and Rudistids. The Mutelidac, Pandoridae, Clavagellidae, 
Poromyacidae, Crassatellitidae, Cryptodontidae, Petricolidae, true Solens, the Mactridae, 
and Saxicavidae, also take their origin during this period. 
With the beginning of the Tertiary a gradual approximation to present conditions 
takes place. The Rudistae have disappeared, the Dysodonts are on the decline, and 
the Teleodesmacean types on the increase. Numerous Anatinacea, Leptonacea, Tridac- 
nidae, Callocurdiidae, Semelidae, Mesodesmatidae, and Myacidae appear. At the close of 
the Eocene, the wide distribution of many types now characteristic of warm, temperate, 
or tropical waters begins to be restricted ; and during the Miocene the faunal boundaries 
of mollusks depending upon temperature conditions are laid down nearly on existing lines. 
The following table indicates more exactly the geological range of the families of 
Pelecypods according to our present information :— 
