OSTEOLOGY, ETC., OP STNGNATHUS PECKIANUS. 629 



as if the whole anterior wall of the cranial cavity were wanting. 

 The interorbital septum (fig. 11,1. S.) is merely membranous, 

 as in the Siluroids and Cyprinoids. 



Immediately in front of the orbits, on either side, is an 

 ectethmoid (fig. 8, Ect. E.), the ossification of the preorbital 

 process of the ethmoidal cartilage. They extend down the sides 

 of that cartilage, and articulate below with the parasphenoid, 

 and above with the frontals. 



At this point or slightly anterior the frontals terminate, the 

 remainder of the rostrum being formed by the ethmoid, with 

 the vomer lying along its under surface. There appear to be 

 no membrane bones occupying the position of the nasals of most 

 fishes. 



The ethmoid (fig. 12, Eth.) consists posteriorly largely of 

 cartilage surrounded by a certain amount of ectosteal bone. 

 In the cartilage, on either side, is a canal, in which the olfac- 

 tory nerves and vessels run, passing to the olfactory capsules, 

 which form deep indentations in the sides of the cartilage. 

 Immediately in front of the orbits the cartilage appears to con- 

 sist of two portions, an upper, originally the tegmen cranii, and 

 a lower, the coalesced trabeculae, and between these the olfac- 

 tory nerves and vessels at first run. This distinctness of the 

 two parts obtains, however, only for a short distance, the two 

 halves soon uniting and becoming indistinguishable, tlie olfac- 

 tory structures becoming enclosed in a canal. In front of the 

 olfactory organs the cartilaginous part of the ethmoid rapidly 

 diminishes, there being a nearly corresponding increase in bone. 

 In this a canal appears, which passes towards the surface as 

 one traces it forwards. I take this to be the continuation of 

 the main slime canals, which also traverses the frontals. Ante- 

 riorly the ethmoid becomes thinner, but remains nearly of the 

 same breadth, and its cartilaginous portion entirely disappears ; 

 still more anteriorly (fig. 13) it becomes almost scale-like, the 

 vomer (Vo.), which hitherto has been round and small, now 

 becoming larger and triangulai*, and forming the greater part 

 of the thickness of the rostrum in this region, and this relation 

 persists to its extremity. 



VOL. XXIII NEW Sl'Jll. T T 



