286 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Mar. 25, 



by the ornament of the dermal covering of the shield. In this 

 species the centres of the polygonal ossicles of the second stratum 

 of the buckler are elevated into distinct umbones, and over these 

 a thin dermal layer is spread, thickly covered by small granular 

 asperities. From the combination of the two characters, a very orna- 

 mental design results, which can be recognized in a mere fragment 

 of the shield (PL IX. fig. 3, 6), and warrants the designation of 

 Cephalaspis 07'natus, which I propose for this species It differs in 

 other respects from the species already described. The orbits are 

 small and more distant from each other than they are in Cephalaspis 

 Murchisoni, In the slight amount of curvature of the posterior 

 margins of the shield, and in the thickening of the margin, it agrees 

 vdth the corresponding parts of Cephalaspis Murchisoni. The di- 

 mensions are as follow °. — from the snout to the posterior edge of the 

 shield 1 inch and nine-tenths, — of this the anterior portion from the 

 snout to a point on the median line between the orbits is eight-tenths, 

 and from thence to the nape eleven-tenths. The exact width across 

 the line of the orbits is not ascertainable : it probably corresponded 

 pretty nearly with the length of the shield. 



These specimens are from a more argillaceous stratum than those 

 above described, and it is possible that its preservative properties 

 may have protected delicate characters such as the skin-ornaments, 

 which have perished in the more arenaceous beds from which the 

 other specimens were collected. The other discrepancies are not so 

 decided but that, should a better-preserved specimen of Cephalaspis 

 Murchisoni reveal the dermal characters above alluded to, the spe- 

 cific name ornatus may be cancelled, and the characters here de- 

 tailed be appended to the specific attributes of Cephalaspis Murchi- 

 soni. 



x^ucHENASPis Salteri, gcu. ct spcc. nov. PI. IX. figs. 4 & 5. 



The same argillaceous beds in the Ludlow railway -cutting from 

 which the specimens last described were derived have furnished 

 two other specimens of great interest. They are both tolerably 

 perfect, and correspond so closely in every particular, that it matters 

 little which is selected for description. What is written concerning 

 one is corroborated by the other. I take then the first that presents 

 itself, andl find a Cephalaspis in miniature, a pigmy not larger than 

 a fourpenny-piece. The outline of the shield, the" eye- sockets, and 

 the nasal depressions, are all clearly defined. If size were a cri- 

 terion of species, it would be undoubtedly a new one. Questions, 

 however, of age and growth (of which we* have as yet had no evi- 

 dence in this group of extinct fishes) have to be duly considered 

 before relative magnitude can be taken as a specific character. 



On examining the specimen more closely, a new feature is observed. 

 Behind the shield, and united to its posterior margin by a distinctly 

 marked suture, is situated a broad plate divided into two lateral 

 halves by a prolongation of the occipital crest (PI. IX. figs. 4 & .5). 

 These plates are nearly coincident in width with the diameter of the 

 shield, and in length equal two-thirds of its antero-posterior dimen- 

 sions. The evidence of the Scotch specimens of Cephalaspis Lyellii 



