320 Geological Society : — 



appearance of sutures on the inner surface of the shield is, indeed, 

 alone sufficient to prove that it is not composed of distinct scales. 



In the shield of Pteraspis three principal layers are similarly- 

 discoverable : the inner is very distinctly laminated ; the outer, 

 almost wholly constituted by the characteristic " enamel- ridges," 

 consists of Kosmine. Vascular canals pass from the inner surface, 

 and ramify in the middle layer, terminating in cseca in the outer layer, 

 as in CephaJasjjis. 



But there are no osseous lacunae ; and the vascular canals com- 

 municate with large polygonal cells (which were either empty, or 

 more or less occupied by membranous substance in the recent state) 

 situated in the inner part of the middle layer. 



Specimens were exhibited in which these cellular cavities were 

 empty ; but ordinarily they are filled with the matrix, which then 

 assumes the form of polygonal prisms separated by the thin walls 

 of the cells. It is these prisms which have been mistaken for part 

 of the bony structure itself. 



On examining a thin section of one of M. Kner's specimens (for 

 which the author is indebted to the liberality of Sir P. Egerton), the 

 structure, though much altered, showed sufficient similarity to that 

 of the specimens of C. Lloydii in the Museum of the Society to leave 

 no doubt as to the generic identity of the two. 



The microscopic examination of Pteraspis demonstrates its un- 

 questionably piscine nature ; and shows that, while in many respects 

 similar to Cephalaspis, the species included under Pteraspis are rightly 

 separated from the others. The leading distinctive characters of 

 the former are the absence of osseous lacuuce, — the cellular character 

 of the middle layer, — and the ridged and not tuberculated enamel. 



In conclusion, tlie author inquired into the evidence of the Ganoid 

 nature of the Cephalaspidce, and into the value of the relative and 

 absolute development of the endo- and exo-skeletons in fishes, con- 

 sidered as indications of the perfection of their general organization. 



2. " On a New Species of Plesiosaurvs ; with Remarks on the 

 Structure of the Atlas and Axis, and of the Cranium in that genus." 

 By Prof. Huxleyj F.R.S., F.G.S. 



The specimen which is the subject of the present paper was pro- 

 cured at Street, near Glastonbury. It is now in the Collection of 

 the Museum of Practical Geology, Jermyn Street, and it will be de- 

 scribed at length in the Decades of the Geological Survey. 



It approaches most nearly to P. Hawkinsii; but the head is 

 smaller in proportion to the body and neck, and the number of the 

 cervical and dorsal vertebrae is dift'erent, there being altogether fifty- 

 three cervico-dorsal vertebrae, of which thirty are cervical ; while in 

 P. Hawkinsii the cervical vertebrae are thirty-one, and the dorsal 

 at least twenty-three. For this species, characterized by fifty- 

 three cervico-dorsal vertebrae, — by a cranium at most not more than 

 •jLth of the length of the body, — and by having the anterior thirty 

 vertebrae full)', or more than, equal to four lengths of tlie cranium, 

 the name of P. Etheridgii is proposed. Its dimensions are nearly the 

 same as those of P. Hawkinsii, its length being between 7 and 8 feet. 



