754 MK. E. S. GOODRICH ON [NoV. 12, 



plates of Cladoselache, and the dermal plates of certain Holo- 

 cephali (29) may also be of this nature, since they appear to be 

 formed of dentine-like substance. 



The Scales and Plates of the Heterostraci. — The impoi-tant 

 researches of Traquair (23 & 24) have disclosed a most interesting 

 series of Palaeozoic fish in which it appears to be possible to trace 

 clearly the evolution of the bony carapace of the Pteraspids from the 

 simple placoid scales of Thelodus. The latter (text-fig. 1 96, c & f) are 

 broad and flattened denticles closely set in a mosaic on the head and 

 trunk, and fitting together by their crenulated edges. In the tail 

 region they lose the crenulations, are set farther apart, and are more 

 spine-like in shape (text-fig. 196, g). Rohon (18) and Rose (21) have 

 described their finer structure. The pulp-cavity is simple, and there 

 is no distinctly marked basal plate. The whole scale is formed of 

 typical dentine. Psammosteus is almost entirely covered with large 

 plates (Traquair, 23). Sviperficially these are studded with small 

 denticles in every way similar to those of Thelodus (text-fig. 196, h). 

 They have a rounded or pointed top, a wider base, and more or 

 less closely-fitting crenulated edges. Below these denticles is a 

 thick plate of bone-like tissue, which, however, is devoid of bone- 

 cells. Since the structure of these plates has not been described in 

 detail I give some figures of sections (PI. XLIII. fig. 4, & text- 

 fig. 196, d). An outer spongy layer, pierced in all directions by 

 a network of vascular canals, occupies the greater thickness of 

 the plate. It is indistinctly subdivided into two by a layer of 

 lamellae parallel with the surface. The inner side of the plate is 

 strengthened with a thick lamellated layer. These skeletal 

 structures have been deposited in successive laminae, as is evi- 

 denced by the striation seen in sections (PI. XLIII. fig. 4). The 

 plate grows in thickness by the addition of new layers on its 

 lower surface. 



The denticles are of quite difierent structure, are composed of 

 true dentine very like that of Thelodus (PI. XLIII. fig. 4), and 

 contain a pulp-cavity. They rest on the underlying plate, to which 

 they become fixed, being fused to it here and thei'e at their base. 

 But they are sufficiently separate to be frequently broken off in 

 specimens. These denticles grow, of course, by the addition of 

 new matter below ; and so the pulp-cavity becomes very shallow, 

 and is not exposed even when the cone is much worn down. 



It is but a step from Psammosteus to Pteraspis^ whose exo- 

 skeleton has been well described by Huxley (9) and Lankester 

 (11). Here the denticles already elongated in some regions of 

 Thelodus have been converted into long narrow, closely fitting 

 ridges (text-fig. 196, i). Pander figures a fragment of a plate, 

 probably of Psammosteus^ which shows most beautifully inter- 

 mediate stages in the elongation of the denticles (14). Each ridge 

 of the Pteraspis shield shows in transverse section the structure of 

 a typical placoid scale, with numerous tubules radiating fi'om the 

 elongated pulp-cavity into the dentine. Very narrow deep valleys 



