OKHKKIII RANGE OV THE PELECYPODA ISI 



the Silurian, and of the-e tin- recent n-piv-entat i\ .- an- everywhere n-cDgni.-ed a- 

 constituting tin- nio-t distinctively archaic type of pelec\ pod structure now exi.-t ing. 



The order Anomalodesmacea i- represented in tin- Palaeozoic solely by it.- i-adical, 

 the /'//"/"'/'///'/"'; eight ,,)' its sixteen familie- originate in tin- Me.-o/ojc and Ti-rt iar\ .' 

 and, with tin- exception of tli.- /'//"/"'/'///'/"rand /'/</'/-,/, ,,i<i<-i<l,i, , all have ciidun-d until 

 tin- present time. Only one family appears to lie exclusively Recent. 



The Teleodesiiuict'-a aiv dist im-t i vely modern, although foreshadowed in the 1'ahn-o- 



/oic by Cypricardian, Lucinoid, and Allodesmid radical.- the Solenoid radical i- .-till 

 questionable). 01' tur1y->-v-n i'aniilies thirty can l.e tii>t detinitely recognir-ed in the 

 Me.-o/ojc, twelve originate in the Tertiary, two are exclusively rer.-ni.and only a -ingle 

 one can lie traced continuously from the Palaeo/oic ID the recent fauna. 



Of the Prionodesmaceau families, 1O5 ]>er cent .-urvive; of the Teleodesmacean 71 

 per cent ; and of the Anomalodesmacean SS per cent. If it wen- not for the mortality 

 amon<,' the dm imn-i'ii and Hii<Iixf<n; the i-atio of sui-vival amon<, r the Teleode.-marean 

 families would Ite !)5 per cent. Of 1O."> families which have been discriminated during 

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

 ex i.-t ing fauna. Families have originated in the various geological epochs as follows : 

 Cambrian 1, Silurian (Murchisonian) 20, Devonian 10, Carboniferous '', 1'ermian -1, 

 Trias 13, Jura 14, Cretaceous 18, Eocene 15, Miocene and Pliocene :j, Pleistnr.-n.- 

 and Kecent 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 number. 



The Silurian is especially characterised by the presence of Taxodont, Palaeoconch, and 

 the older formsof Schizodont Pelecypods. The P r lastidae^Cardiolid<',Anfiii/''iiridae,Cteno- 

 //'/>, /tiiln,', I'f<-riii<'i<l<ti', Ambonychndae, and Modiolopsidae are shared with the Devonian. 



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

 diniidae, the Merjalodontidae, Trigoniidae, Pinnidae, Pectinidae, and Mytilidae fii-st 

 take rise in 1hi> period, and the sinupalliate Allorisma is the first Pelecypod showing 

 clear evidence of retractile siphons. 



The Carboniferous is marked by the appearance of Parallelodvn and its allie.-, 

 the Li'iiiiilin- and Ostreidae, and some precursors of the Lucinacea and /'/"//<"/'"> ". 

 The /\/-///'/"' and GastrochaeniJn make their advent in the Permian ; but, on tin- 

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

 important changes take place; many old genera disappear, and such forms a> A///"-y- 

 sidae, the ti-ue I'ninn^ SpondylvA, /'//////", the Pleuromyaadae^ Pholadomyacidae) 

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



During the Jura, genuine Arcidae, Anomta, Eligmus, vsirioua Anatinacea, Cyrma, 

 /'/c,/v/x, /,s,;r" /'//", and the Teleodont Veneridae, Tellinidae, Donacidae, and /'/'"/"'/"'"' 

 are initiated. The character of the Cretaceous is strongly influenced by the aberrant 

 and short-lived Chamacea and Kudistids. The. Mutelidae, Pandoridae, Clavagellidae, 

 Poromyacidae, Crassatellitidae, Cr>n>t<>"n l ,>nlae, Petricolid<', true Solens, the Murtridn,, 

 and Saxicarnlin; also take their origin during this ]>eriod. 



With the beginning of the Tertiary a gradual approximation to pn-eiit conditions 

 takes place. The Rudistae have disappeared, the Dysodonts are on the decline, and 

 the Telcodesmacean types on the increase. Numerous Anatinacea, Leptonacea, Tr!<l<n-- 

 nidae, Callocardiidae, Semelidae, Mesodesmati(/<i<, and Myacidae ap})ear. 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 tempera! ure conditions are laid down nearly on existing line.-. 



The following table indicates more exactly the geological range of the families of 

 Pelecypods according to our present information : 



