462 MR. W. C. WILLIAMSON ON THE MICROSCOPIC STRUCTURE OF THE 
what appear to be lepidine tubes, ascending from the inferior surface to the upper- 
most layers of the structure, fig. 26 h. 
The general aspect of the exterior of the body of Macroipoma Mantelli, is that of 
an ordinary scaled fish. This is especially shown in a fine specimen, formerly in the 
collection of Dr. Mantell, and now in the British Museum ; originally figured by 
that gentleman in his work on the Fossils of the South Downs, and afterwards by 
M. Agassiz*. Specimens exhibiting the exterior of the fish are very rare, since, 
owing to the roughness of the outer surface of the scales, they are usually adherent 
to the matrix, the inner portion being exposed ; or, what is even more frequently the 
case, each scale has split horizontally, and only exhibits its internal tissues. 
According to the enlarged figures of M. AGASSiz-f, the anterior part of the upper 
surface of each scale is marked with concentric lines, which he regards as lines of 
growth ; whilst the posterior or visible portion is crowded with elongated tubercles, 
or pointed cylinders ; those on the centre of each scale being the largest. On making 
a very careful examination of the surface of these scales, I found that the tubercles, the 
“cylindres pointus” of Agassiz, were dermal teeth, corresponding with those already 
described as existing on the opercular scale; instead however of being nearly orbi- 
cular as in that example, they are all more or less elongated, whilst some of them, 
and especially four or five large ones ranged along the middle of many of the scales, 
stand up in bold relief, appearing like well-defined pointed teeth, equal in their degree 
of development to any which I have seen in the skins of recent Placoids. The pulp- 
cavity is similarly elongated, still however giving off the radiating canals at its base. 
These latter frequently communicate between one cavity and another. 
Each of the teeth is irregularly grooved on its external surface, and these grooves, 
being prolonged in irregularly parallel lines on the broad thin expansions into which 
the bases of the teeth spread out, give to them somewhat the appearance of con- 
centric striae. In none of those which I have seen have I been able to discover the 
regularly concentric arrangement shown in the figures of M. Agassiz. Plate XLIII. 
fig. 2/ represents a vertical section of the greater part of a scale in which the tubercles 
are very small, but in which the lower tissues appear to be complete; 27/* is the 
posterior extremity of one scale resting upon the anterior margin, 27 g of another; 
a small portion of the posterior extremity of the latter is still wanting. 
The textures exhibited in this section are divisible into two portions, an upper and 
a lower one. The former, fig. 27 «, is of a dense structure, and appears mainly to con- 
sist of the expanded bases of the tooth-like tubercles in which some few lacunae are 
developed. In this the tubercles are implanted, 27 h, each of which corresponds in its 
general structure vvith those already described from the opercular scale, fig. 26 a, only 
in this instance they are more depressed, and the pulp-cavity is larger in proportion to 
the size of the tubercle. The concentric lamellae of the kosmine are also seen com- 
pletely to surround the pulp-cavity, being continuously developed to some extent be- 
* Poissons Fossiles, vol. ii. tab. 65 b. f JJt supra, tab. 65 b. 
