596 



FISHES OF THE OLD RED SANDSTONE. 



establishing species, in order to avoid the chance of considering as peculiar species those specimens 

 where the exterior layer having been destroyed, the surface of the disks exhibit a totally different 

 aspect. Similar mistakes have already been made in the class of Crustacea, among which the 

 inferior surface of the shell has frequently a different aspect from that of the superior, so that their 

 casts or impressions have no resemblance to each other." (See Chapter on Trilobites.) 



The Cephalaspis Lloydii occurs in several situations in the cornstone of Herefordshire, Shrop- 

 shire, Worcestershire, &c, and in the same localities with the C. Lyellii. It has with great pro- 

 priety been dedicated to Dr. Lloyd of Ludlow, who first made known to me the existence of fishes 

 in the Old Red Sandstone, see p. 180, and whose collection has supplied the greater number of the 

 forms here described. 



Ichthyodorulites. 



"The Ichthyodorulites of the Old Red Sandstone," observes M. Agassiz, "belong to distinct 

 species of the genera Onchus and Ctenacanthus, some species of which exist in your Carboniferous, 

 Old Red and Silurian Systems. These bony spines are more or less arched, and grooved by longi- 

 tudinal furrows, separated by round ridges forming ribs. In the greater number of the species, 

 these ridges cover the whole surface of the fins : there is, however, one species of Onchus, the 

 posterior side of which is partly smooth. The great distinction between this genus of Ichthyo- 

 dorulites, and the large species of Hyhodus of the lias (and New Red Sandstone) \ to which they 

 have some resemblance in the arrangement of their longitudinal furrows, is, that their posterior edge 

 has no sharp points (piquans) or teeth, while in the genus Hyhodus there are on that side strong 

 points which are arched downwards." 



Onchus arcuatus, Agass. (PL 2. figs. 10 and 11.; Recherch. vol. iii. t. 1. figs. 3, 4 and 5.) 



" This genus is easily distinguished. It embraces certain cartilaginous jishes, the dorsal spines 

 of which only have yet been discovered. 



"Dorsal fins large at their base, very much bent backwards, narrowing rapidly towards the supe- 

 rior end, furrowed along the whole of their surface by grooves parallel to the posterior margins, 

 between which pretty strong ribs rise up, which so terminate as to give the anterior edge a toothed 

 aspect. It is the largest species of the Old Red System, though inferior in size to some species of 

 the Carboniferous Limestone." 



Onchus semistriatus , Agass. (PI. 2. figs. 12 and 13.; Recherch. vol. iii. t. 1. f. 9.) 



" Characteristic though imperfect fragments," observes M. Agassiz. "The posterior portion of 

 the lateral faces of these fins is smooth, but the anterior part is grooved with numerous longitudinal 

 furrows, between which are delicate flattened ridges. As the other species of Onchus are channeled 

 all over, this form may, after all, belong to a distinct genus, but the imperfect condition of the 

 specimens I have seen does not permit me to establish it." 



1 A dorsal spine of Hyhodus has recently been found in the Keuper Sandstone of the red marl of England, 

 which, appearing to differ from any published species of Agassiz, Mr. Strickland and myself have called ". Hy~ 

 hodus Keuperi." — See Proceedings of the Geological Society, vol. ii. p. 563. 



