340 GEOLOGICAL SURVEY OF THE TEREITOEIES. 



•witli the pectoral. lu the rays found together, the first only had a 

 trenchant outer margin, while several had a rabbett along one side of 

 the posterior margin. I have already described such a spine as per- 

 taining to the pectoral fin of IcMhyodectes 'prognathus. 



The vertebrae in all the species certainly assignable to this group are, 

 where known, deeply two-grooved on each side, besides the pits for the 

 insertion of neurapophyses and pleurapophyses, except in the cervical 

 region, where the lateral grooves are wanting. There are no diapophyses. 

 The caudal vertebrae are rather numerous, but not so much so as in 

 Amia, nor are they so mucih recurved as in that genus. 



Until the structure of the posterior cranial roof and of the scapular 

 arch are fully made out, it is premature to state precisely the affinities 

 of this family. So far as known, they are Isospondyli, with some char- 

 acters of the Salmonidce, and some of other significance. The large 

 foramen behind the prootic bone is more Clupeoid in character. The 

 femoral bones are more like those of the Flectospondyli, dividing, in a 

 measure, characters of the Gyprinidw with those of the Mormyridce. 

 The vertebrae are Clupeoid. while the mode of implantation of teeth is 

 peculiar. 



Synopsis of genera. 



I. Jaws without foramina on the inner face of the alveolar margin: 



Teeth of unequal lengths in the maxillary and dentary 



bones Portheus. 



Teeth of equal lengths, cylindric , IcMhyodectes. 



II. A series of foramina on inner side of alveolar wall: 



Teeth with subcylindric crowns Saurodon. 



Teeth with short compressed crowns Saurocephalus. 



There are some other forms to be referred to this family whose charac- 

 ters are not yet fully determined. Thus, Hypsodon, Agass., from the 

 European chalk, is related to the two genera first named above ; but as 

 left by its author in the '^Poissons fossiles," includes apparently two 

 generic forms. The first figured and described has the mandibular teeth 

 of equal length. In the second they are unequal, as in Portheus, to 

 which genus this specimen ought, perhaps, to be referred. Both are 

 physostomous fishes, and not related to the Sphyrcenidw, where authors 

 have generally placed them. Eetaining the name Hypsodon for the 

 genus with equal mandibular teeth, its relations to IcMhyodectes remain 

 to be determined by further study of the H. levesiensis 



A species of IcMhyodectes from the chalk of Sussex, England, is figured 

 but not described by Dixon in the Geology of Sussex. 



A number of forms erroneously referred by Agassiz and Dixon to the 

 genus Saurocephalus have been referred by Leidy to a genus he calls 

 Protosphyrcena,* with two species, P. ferow and P. striata. The latter 

 much resembles a Saurocephalus, having equal teeth, while the former 

 probably includes several species and possibly genera. The teeth first 

 referred to it resemble generically those of P. striata, while others resemble 

 those of Portheus. An examination of the figures of the mandibles of the 

 last, in Dixon's work, shows that the large and small teeth occupy differ- 

 ent areas, separated by grooves, in a manner quite distinct from any- 

 thing seen in Portheus ; but should it prove identical, it can scarcely be 

 regarded as typical of Protosphyrcena, which name, moreover, has never 

 been accompanied by the necessary description. 



* Trans. Amer. Philos. Soc, 1856. 



