CEPHALASPIDtE 7 



Dr. Rudolph Kner describes the structure of his Pferaspis from Gallicia as follows : — 

 " The form of the fossil is very similar to that of C. Lloydii ; but it is larger, having a 

 length of about four inches by a width of two. It consists of three layers. The innermost 

 is shining, bluish-green, enamel-like, and presents four or five distinct lamellas. This layer 

 forms one continuous surface, marked in the centre by a longitudinal depression smaller 

 at one end than the other, and by obscure radiating lines. The upper part of the conical 

 depression is covered with minute pores or depressions, wbich are visible in the deeper as 

 well as in the more superficial layers, but become evanescent in its lower part. Between 

 the layer of enamel and the prismatic part which succeeds it, there lies a thin dull layer, 

 in some places of a brownish colour. This is followed by an excessively delicate lamina 

 of enamel which lies upon the prisms. The layer of prisms is one line thick, and in sec- 

 tion presents a number of more or less hexagonal discs. The enamel passes for a short 

 distance between the prisms. Externally the prisms lie on a granular layer, to which the 

 outermost very delicate epidermic lamina marked with parallel striae succeeds." Such was 

 the structure which Dr. Kner believed he had seen in his Pteraspis, and compared with 

 that of the Cuttle-bone, to which it really presents but a superficial resemblance. It was 

 on account of this structure that Dr. Kner proposed to remove Cephalaspis Lloydii and 

 C. Lewisii from the Class of Fishes, and place them with his new form as Pteraspis among 

 the Cephalopodous Mollusca. 



Professor Pander describes briefly the structure of the Russian Cephalasjnda {Thyestes) 

 in his work already quoted, from which it is evident that they have very little in common 

 with Pteraspis. Sections of tubercles from the margin of the shield exhibited " a homoge- 

 neous base, in which clear and dark cells of the most various forms (rounded, elongated, 

 and angular, with fine radiating branches) lay scattered, and were frequently disposed in 

 concentric layers, where a tubercle rose above the general surface. Although they have 

 not the same general form as ordinary bone-lacuna? (such as occur in Pterichthys and 

 Coccosteus), yet they can hardly be called by any other name." 



Professor Huxley, from whose memoir the above account of his predecessors in this 

 inquiry is chiefly quoted, has left very little to be added to his exhaustive description of the 

 structure of the shield in Cephalaspis Lyellii and C. Lloydii. I have examined the struc- 

 ture in Cephalaspis Murchisoni and in C. [Pteraspis) rostratus, and have nothing of 

 importance to add, but am able to confirm his description from the examination of a very 

 large number of specimens. Taking then, first, Cephalaspis Lyellii and its allies repre- 

 senting the OsTEOSTRACi, we will return afterwards to Cephalaspis Lloydii, C. rostratus, 

 &c. (Kner's Pteraspides), typical of the section Heterostraci. 



A naked-eye examination of the shield of Cephalaspis Lyellii shows in most specimens 

 that deceptive apparent division of the outer surface into polygonal scales described by Pro- 

 fessor Agassiz. In some curiously preserved specimens of a species from Scotland I have 

 observed the divisions really existing, the upper surface of the shield being split up into 

 polygonal pieces like a tesselated pavement. In other species, however, from England 



