VERTEBRATES. 4G3 



hooked, closely approximate deiiticles, which might at first glance be 

 mistaken for a continuous sharp enameled ridge, but which under the 

 lens are seen to be isolated though very closely arranged denticles, 

 tbeir abrupt inferior acclivity rising into a delicate transverse carina, 

 the long superior slope culminating in a sharp ridge, as shown in figure 

 3 c, d. "Whether the approximate arrangement of the denticles observed 

 in the above specimen is indicative of specific distinctness from its asso- 

 ciates we have not the means of determining; it is, however, probable 

 that these closely arranged denticles gradually merge into the widely 

 spaced and finally obtuse tubercles occuriug near the base of the spine, 

 as shown in fig. 1 a, and fig. i a. 



In nearly all specimens the strong anterior ridge of the posterior 

 spine is distinctly shown, though the lateral costa? may be completely 

 destroyed, and often so compressed basally as to appear like a flat scale 

 without the least indication of internal cavity, presenting in transverse 

 section a wedge-shaped outline, the broad extremity formed by the 

 anterior ridge. But an otherwise much worn individual obtained by 

 one of the writers at Alton, and the first example of the geuns found 

 in our rocks, fortunately shows the entire section of both arms, though 

 the base as usual is flatly compressed. In the latter specimen the pos- 

 terior spine in the upper third has an ovate transverse section, the 

 pulp cavity occupying about one-third the diameter, and subcentral or 

 situated within the posterior two-thirds of the diameter; it doubtless 

 rapidly expands infeiiorly, where the comparatively thin walls offer 

 slight resistance to pressure exerted from without, as is the case with 

 the basal region. The anteriorly produced appendage is traversed 

 throughout by the internal cavity, which is continuous with that of the 

 posterior spine and basal region, inclosed interiorly by very thin walls, 

 but diminishing towards the extremity, where the pulp cavity is about 

 half the diameter of the body of the spine, and closely crowding the 

 superior (posterior) wall, while along the inferior (anterior) side the 

 inclosing walls are relatively massive. From these facts alone, even in 

 the absence of any knowledge of the direction of the inserted base in 

 relation to the external processes, the relative position of these append- 

 ages would be readily inferred from the posterior position of their 

 internal cavities. 



Intimately allied to E. Jonesii, McCoy, of the Carboniferous limestone 

 of Armagh, Ireland, it is chiefly distinguished by the more restricted 

 posterior extension of the tuberculose surface, which in that form 

 extends up the posterior spine a third of the distance from its base, and 

 the less numerous costse which reach to an oblique line descending from 

 the angle of divarication. Whether or not the apparent dissimilarity 

 in the transverse sections of these spines is real, or that the flattened 



