198 FISHES. 



species of Acrodus and Strophodus. Of tlie Ganoid genera 

 Gcdacanihus, Amhlypterus (Palceoniscidm), Saurichfhys persist 

 from the Carboniferous epock Ceratodus appears for the first 

 time (Muschel-Kalk of Germany). 



Thanks to the researches of Agassiz, and especially Sir P. 

 Egerton, the ichthyological fauna of the Lias is, perhaps, the 

 best known of the Mesozoic era, 152 species having been 

 described. Of the various localities, Lyme Eegis has yielded 

 more than any other, nearly all the Liassic genera being 

 represented there by not less than seventy-nine species. The 

 Hybodonts and Cestracionts continue in their fullest develop- 

 ment. Holocephales (Isckyodus), true Sharks (FalceoscyUium), 

 Eays (Squaloraja, Arthrojpterus), and Sturgeons (Chondrosteus) 

 make their first appearance ; but they are sufficiently distiact 

 from living types to be classed in separate genera, or even 

 families. The Ganoids, especially Lepidosteoids, predominate 

 over all the other fishes : Zejoidotus, Semionotus, Fholidophorus, 

 Pachycormus, Uugnathus, Tetragonolepis, are represented by 

 numerous species; other remarkable genera are Aspido- 

 rhynchus, Belonostomus, Saurostomus, Sauropsis, Thrissonotus, 

 Conodus, Ptycholepis, Undactis, Centrohpis, Legnonotus, Oxygna- 

 thus, Heterolepidotus, Isocolum, OsteorhacMs, Mesodon. These 

 genera offer evidence of a great change since the preceding 

 period, the majority not being represented in older strata, 

 vi'hilst, on the other hand, many are continued into the suc- 

 ceeding ooHthic formations. The homocercal termination of 

 the vertebral column commences to supersede the heterocercal, 

 and many of the genera have well ossified and distinctly seg- 

 mented spinal columns. Also the cycloid form of scales 

 becomes more common: one genus (Leptolepis) being, with 

 regard to the preserved hard portions of its organisation, so 

 similar to the Teleosteous type that some Palceontologists 

 refer it (with much reason) to that sub-class. 



[See E. Sauvage, Essai sur la Faune lolithyologique de la periode Liasique. 

 In "Bibl. de I'ecole des hautes etudes,'' xiii. art. 5. Paris 1875. 8".] 



