FOSSIL FISHES. 27 



The departure from this structure exhibited by the scaled body of the 

 Lepidosiren will suggest itself at once as a marked point of difference 

 between them, but we find great diversity in the dermal defenses of 

 fishes even within the same family ; for example, Pterichthys and Coo- 

 costeus seem to have been closely allied, yet the hinder portion of the 

 body in one was scaled, in the other naked. 



Again, in the living Siluroids most are without scales or plates, the 

 body being protected, as in our cat-fishes, only by a leathery skin ; yet, 

 as we have seen, some genera of this family, as Arius, Bagrus, etc., have 

 the region of the vital organs protected by large bony plates. 



A similar diff'erence seems to prevail between the ancient and modern 

 representatives of the genus Ceratodus. No traces of scales have been 

 found with the teeth of the Triassic species, wliile the living Ceratodus 

 of Australia has the body covered with large scales, like those of Lepido- 

 siren. 



DnsricHTHYs Terrblli, ISTewb. 



Charts V. and VI. 



Biniclaliys Hertzeri, N., in part. Vol. I., Part II., p. 316, PI. 32, 33, 84. > 



Diniohthys Terrelli, N. , Vol. I., Part II., p. 3^3, note. J fc !► / / 



As indicated in the above references, this species was named in a note 

 appended to the description of Diniohthys Hertzeri in our first volume on 

 Palaeontology. On the preceding pages some of the bones belonging to 

 this fish were referred to as parts of D. Hertseri, and some of the figures 

 given to illustrate the text were ascribed to that species ; whereas, in fact, 

 they represent bones of D. Terrelli. This confusion of the two species 

 was occasioned by the- circumstance that the most complete specimens 

 found by Mr. Hertzer, at Delaware, consisted of the jaws and teeth, 

 and for a long time almost nothing was known of the plates of the head 

 and body of the Delaware species. Singularly enough, it happened that 

 all the specimens found on the Lake Shore by Prof. Allen and Mr. Ter- 

 rell during two years subsequent to the discovery of the first bone there, 

 were cranial and body plates. Hence, the material for the diagnosis of 

 the species was not in my possession, and all the bones from both localities 

 were ascribed to D. Hertzeri. 



The most characteristic features of Diniohthys Terrelli have been inci- 

 dentally mentioned on the preceding pages ; no extended description of 

 the species is therefore required here. A concise review of its structure 

 is, however, desirable for its more accurate definition, and to bring out 

 the characters in which it differs from D. Hertzeri. Such a review is 

 given below. 



