338 FOSSILS FROM THE 



The interior head of the unnamed ganoid whose auricular 

 organs we have just examined, resembled, we have seen, that 

 of a shark ; the interior head of another and better known 

 ganoid of the same formation, — the Dipterus, — seems to have 

 resembled that of the sturgeon. It was covered externally 

 by strong osseous plates, but occupied within by a continuous 

 mass of cartilage, in the middle of which we find the brain- 

 pan scooped out like a cell in a sandbank, and the cerebral 

 portion of the spinal cord communicating with it through a 

 conical-shaped tube bent upwards, and wide at the nape, but 

 narrower where it enters the brain-chamber. I have else- 

 where compared this conical-shaped tube to the interior of a 

 miniature buglehorn ; a second speciman, which I disinterred 

 last autumn from among the rocks of Thurso [Spec. 8], exhi- 

 bits a cross section of this spinal tube, and indicates the up- 

 ward bend from a different point of view. In the unnamed 

 ganoid the bend of the spinal tube seems to have been directed 

 downwards. 



Let me next remark, that the base of the skull differs 

 greatly among the different fishes which still inhabit our seas. 

 It is flat in the rays ; less flat in the sharks ; while in most 

 of the osseous fishes it exists as a narrow ridge, composed of 

 three bones, — the base occipital bone, the sphenoid bone, and 

 the vomer. In most of the ordinary fishes, the sphenoid bone 

 is a mere beam, and it assumes nearly the same beam-like 

 form in one of the most characteristic of existing ganoids, 

 — the Lepidosteus. Now, among the ganoids of the Old 

 Red Sandstone we find cranial bases of both the broad and 

 the narrow type. I stated, in my little work on the Aste- 

 rolepis, that I had no better evidence that some of the in- 

 ternal bones figured belonged to that ichthyolite, than that 

 they occur in the same beds with the dermal plates, which 

 bear the characteristic star-like markings, — that they are of 

 considerable size, — nn<T tnax tiiey formed no part of the known 



