Mesozoic and Ccenozoic Geology and Palaiontolo(jy. 243 



tence to recent times, except Spirifera, which, so far as known, termi- 

 nated its career in the Jurassic age. 



The Gasteropoda is represented bj nine species belonging to eight 

 genera. Two of these genera, Dentaliinn and Turbo, had an existence 

 in the Palaeozoic age, and continued to live until the Tertiary period. 

 One genus Tdoplacodes commenced and terminated during the age in 

 question. The other five genera, JSTeriteUa, Neritina, Planorbis, Val- 

 vata and Viviparns are counted among the living Gasteropoda. 



The Cephalopoda is represented b}- thirty species distributed among 

 seven genera. One, the Nautilus, is a palaeozoic and living genus. 

 Two, Goniatites and Orthoceras, are palaeozoic genera that closed their 

 existence in the Jurassic age. One, Meekoceras, is confined to rocks of 

 the age in question. The other three genera. Ammonites, Belemnites 

 and Ceratites, commenced their existence in the age in question, and 

 terminated their career in the Cretaceous period. 



The Lamellibranchiata is reiDresented by 125 species distributed 

 among 51 genera. Six genera, Avicula, Cardium, Lima, Ifytilus, Os- 

 trea and Pinna, are reckoned among the palaeozoic and living. Of the 

 other 45 genera, eleven of them are palaeozoic, but only 24 have yet 

 been found in the Cretaceous. 19 of these are Tertiary, and 7 are living, 

 all of which are marine except Unio, which is now a fresh-water genus. 

 Or looking at this most numerously represented class of the Inverte 

 brata in another light, we obserA^e that of the 51 genera represented in 

 the rocks in question, 13 genera, or more than 25 per cent., are still liv- 

 ing. 21 genera had passed away before the Cretaceous period, leaving 

 30 genera only in the latter period; and consequentl}' only 17 of these 

 genera have expired since the dawn of the Cretaceous. 



The vast changes in the vertebrate kingdom during this period, and 

 the grand passage from the Batrachia to the IMammalia, evidences the 

 same great laspe of time that is indicated by other organic remain, 

 and inferred from the vast thickness and extensive distribution of the 

 strata. 



The class Pisces is represented by fifteen species belonging to nine 

 genera. Two of these genera, Amblyptemis and Paloioniscus, are also 

 of Palaeozoic age. The other seven are not represented so far as 

 known in rocks of older or younger age. 



The class Aves is represented only hy Paloeonornis struthionoides, 

 a bird named by Prof. Emmons, in 1857 — a genus, however, not yet 

 clearly defined or understood. 



The class Reptilia is represented by 41 genera, none of which are of 

 Palaeozoic age, and only two, Lcelaps and Pterodactylus, are said to 



