FISHES OF THE OLD RED SANDSTONE. 



597 



Ctenacanthus ornatus, Agass. (PL 2. f. 10. ; Recherch. vol. iii. t. 2. f. 1.) 



t£ This fragment," says M. Agassiz, " is probably derived from the middle of a dorsal fin. Im- 

 perfect as it is, the characters of the fins of the genus Ctenacanthus are easily distinguished in 

 the furrows and longitudinal parallel ridges, with transverse folds. This species differs from its 

 congeners in the Carboniferous Limestone {Ctenacanthus major, C. tenuistriatus, C. hrevis) by 

 the tenuity of the longitudinal ridges, which lie very near each other, and by the delicacy of the 

 transverse folds. The interior cavity of the fin appears, proportionably, very large, and the sur- 

 rounding walls more slender than in the other species." 



Locality, near Sapey, Worcestershire : found with other ichthyodorulites by Mr. H. E. Strick- 

 land ; also north of Abergavenny where I have myself observed it. 



Ptychacanthus ? Agass. (PL 1. figs. 9 and 10.) 



It is with great doubt that the fragment, f. 9., of which f. 10. is an enlarged representation, is 

 referred to the genus Ptychacanthus, Agass. " If this form should eventually be ascertained to 

 belong to Ptychacanthus, still," says M. Agassiz, "the species is quite distinct, and unlike those 

 of the Carboniferous Limestone." The fragment is supposed to be the portion of a dorsal fin, 

 because its transverse section exposes a central cavity. 



The above ichthyodorulites, or portions of them, are sometimes abundant where few or no speci- 

 mens of the head of Cephalaspis can be detected. I have observed them in various parts of Here- 

 fordshire, near Abergavenny, in Brecknockshire, and also in the north-western district of Wor- 

 cestershire, 



Structure of Ichthyodorulites. 



M. Agassiz conceives that these fossil-dorsal spines belonged to cartilaginous fishes, the other 

 portions of the animal having been destroyed. Some doubt has recently been thrown upon this view 

 by Mr. Connell of Edinburgh, who, having analyzed these fossils from the limestone of Burdie 

 House near Edinburgh, has found in them the same constituents as in the bones of the osseous 

 fishes, notably of the pike. — Trans. Roy. Soc. Edin., vol. xiii. 



In writing to me concerning this analysis, M. Agassiz thus expressed himself : " So far from 

 fearing that I have misunderstood the nature of Ichthyodorulites, in referring them to cartila- 

 ginous fishes, instead of viewing them as fins of Batistes and Silures, as was formerly the case, 

 I believe that the result obtained by Mr. Connell is a confirmation of my opinion ; at least the 

 texture (^contexture') of the spines of Spinax and Trygon which I have examined with this ob- 

 ject, coincides completely with that of the bones of the bony fishes, and differs in consequence from 

 the other portions of the skeleton of those fishes which are cartilaginous. It would, therefore, be 

 very important to be assured by chemical analysis, whether the spines of the existing cartilaginous 

 fishes {Spinax, Trygon, Sfc.) have also the same composition as the fossil Ichthyodorulites." 



To determine this point, so important in the classification of M. Agassiz, I procured from Mr. 

 Yarrell a serrated spine of one of the rays of a cartilaginous fish, of which he says, "whether this 

 belongs to a species of Trygon, or to a species of one of the allied genera Myliobatus or Cephalo- 

 pterus, all three of which are furnished with similarly serrated spines, I am unable to determine." 



4 G 



