273 GEOGEAPHICAL AND GEOLOGICAL DISTRIBUTION. 



pertains to the first considerable development of the Monomyaria 

 (Pectinidse, Limidae). Here, too, if we except the still problemati- 

 cal Prasostrea, we meet with the earliest indubitable remains of 

 the oyster (Ostrea). As in the case of the Gasteropoda and Brachio- 

 poda, the number of Permian species is very limited. The lamelli- 

 branch fauna of the lower and middle divisions of the Mesozoic 

 series, Trias and Jura, is characterised by a remarkable develop- 

 ment of the families Ostreidse, Pectinids, and Limidse among the 

 monomyarians, the Mytilidse and Pinnidae among the heteromyarians, 

 the Arcadse among the Asiphonida, and the Astai-tidas, Lucinidae, 

 and CardiidsB among the integropalUate Siphonida. The Phola- 

 domyidaB are especially abundant in the Jurassic deposits. Of the 

 two most distinctive Cretaceous groups, the Chamidae and the 

 Rudistae, only the former have their Jurassic representative (Dice- 

 ras). Beyond the great specialisation of those two families, the 

 Cretaceous fauna does not differ essentially from the Jurassic, al- 

 though the number of true sinupalliate forms (Veneridae, Tellinidse, 

 SolenidsB, Glycimeridse) is very much greater. 



Through the different divisions of the Tertiary, beginning with 

 the Eocene, we see the gradual development which by almost im- 

 perceptible stages leads up to the fauna of the present day. The 

 Monomyaria, which in the Mesozoic period constitute nearly thirty 

 per cent, of the entire lamellibranch fauna, enter upon their decline, 

 and are succeeded, as well as the Heteromyaria, by the Dimyaria, 

 of both the sinuate and non-sinuate types. The relation of extinct 

 to recent forms in the different divisions of the Tertiary holds much 

 the same as with the Gasteropoda. 



CRTJBTACEA GENERALLY. 



Of the recent orders of Crustacea the only ones that acquire 

 any geological significance are the Phyllopoda, Ostracoda, and 

 Decapoda, although representatives of some of the other orders 

 occur sparingly in formations extending as far back as the Devo- 

 nian period (Prsearcturus, among the isopods [?]), and possibly 

 even to the Silurian (Necrogammarus, amphipod). It is a rather 

 surprising fact that of the first two orders, if we except one or 

 two special types — Leperditia, Beyrichia — all the most abundantly 

 represented genera of the earliest periods, as well as of the periods 

 succeeding, are such as still hold considerable prominence at the 



