1849.] EGERTON ON THE GANOIDEI HETEROCERCI. 3 



of the dentition of this family with other fishes having blunt rounded 

 teeth, especially with Lepidotus and Tetragonolepis, shows that there 

 is so great a difference in the arrangement of the teeth in the two fami- 

 lies, that even without the test of microscopic examination, the true 

 affinities of the fish can be determined. In the form and arrangement 

 of the scales and in other important respects, Eurynotus differs entirely 

 from the PycnodontidcB and agrees with the Lepidoidei. It certainly 

 resembles Platysomus in the depth of the body and extent of the 

 dorsal fin, but the former character is also common to Amblypterus, 

 and the structure of the dorsal fin is more in conformity with that of 

 this organ in Palceoniscus. The discovery of the dentition of Eury- 

 notus is not without interest, inasmuch as it tends to invalidate the 

 dental characters assigned by Agassiz to the Lepidoid family, and to 

 show that although these fishes may have had " Dents en brosse sur 

 plusieurs rangees, ou une seule rangee depetites dents obtuses *," yet 

 that some of these had (perhaps in addition to those described by 

 Agassiz) teeth of a larger and more massive character. Thus in the 

 ^enxxs, Amhlypterus the teeth are described in the * Poissons Fossiles' 

 as "dents en brosse extremement finesf," while Goldfuss says "dass 

 die Zahne nicht biirstenformig sondern stark und kegelformig sindj." 

 Some of the Palceonisci also have teeth which cannot be considered 

 as "dents en brosse," especially the American species, now arranged 

 under the genus Ischypterus. It is therefore not unlikely, that as in 

 some of the homocerque Lepidoids we find teeth of various forms and 

 sizes associated in the same individual, so a similar condition m.ay 

 have obtained in their heterocerque predecessors. Of the three species 

 oi Eurynotus described by Agassiz, one, viz. Eurynotus tenuiceps from 

 Massachusetts, is considered by Mr. W. C. Redfield to be an imperfect 

 s^QcmiQn oi Palceoniscus {Ischypterus) latus§. 



Genus Plectrolepis, Ag. 



Agassiz in his 'Tableau General' mentions a genus under this de- 

 signation from the coal-measures of Carluke. I have not seen the 

 specimens on which it was founded, but Lord Enniskillen has a fish 

 from the same locality which represents so truly the character ex- 

 pressed in the title, that I feel little hesitation in referring it to this 

 genus. In size and form it resembles a moderate-sized Palceoniscus, 

 but it differs widely in other respects from that genus. The scales 

 are very solid, and covered by a coat of dense, highly lustrous ganoine. 

 The posterior margin is armed with four or five sharp and strong 

 spines, whence the generic name. The bones of the head are covered 

 with coarse wrinkles ; the teeth are blunt, resembling those described 

 above as having been found by Mr. Miller in Eurynotus crenatus. 

 The dorsal fin is situated very far forward, nearer to the head than in 

 any genus of this family. The upper lobe of the tail is covered by a 



* Poiss. Foss. vol. ii. p. 1. t Vol. ii. p. 30, line 23. 



X Beitrage, p. 20. 



§ Short notices of American Fossil Fishes ; read before the Yale Nat. Hist. Soc. 

 April 28, 1841. 



S 2 



