Notes on a Feio Sihirian Fossils from Ayrshire. 171 



somites, nine or ten, probably the latter, with a narrow pro- 

 minent convex axis, gradually decreasing in width towards the 

 pygidium ; lateral portions or pleurte, horizontal, traversed by 

 a nearly central groove, and produced laterally into long, 

 recurved, denticulated spines, those of the last segment (that 

 next the pygidium) bent down almost in a parallel direction 

 with the limb spines of that division of the body — no 

 trace of surface ornamentation preserved. Pygidium short, 

 semi-circular, surface reticulate ; axis of two segments ; limb 

 produced into fifteen sub-equal radiating denticulated spines; 

 the anterior axis segment gives off a ridge on each side, con- 

 tinuous with the anti-penultimate spines on each side the 

 central one of the pygidium ; the denticles are sub-alternate 

 on each side of contiguous spines. 



Ohs. — All the specimens of this species contained in 

 Mrs Gray's cabinet are more or less fragmentary, with the 

 exception of that represented by Fig. 6, a cast, and even 

 here the specimen is a good deal crushed, and the characters 

 of the carapace obliterated. So far as present observation 

 has enabled me to judge, the spines projecting from the 

 anterior part of the carapace are simple, whilst those of the 

 thorax and pygidium are unquestionably denticulated. 



Amongst British species, A. Grayce must be first compared 

 with three species of Acidaspis from Girvan, described by Sir 

 Wyville Thomson, F.E.S.* The form and general proportions 

 of the thorax and pygidium closely resemble those of two of 

 these, A. lalage, and A. liystrix, but the characters of the 

 carapace, so far as they can be made out, do not correspond 

 particularly in the absence of any cervical spines in A. Grayce. 

 The form of the carapace much more closely approximates to 

 that of Sir Wyville Thomson's third species, A, callipareus, of 

 which, unfortunately, the thorax and pygidium are unknown. 

 A. Grayce undoubtedly differs from A. lalage in its denticu- 

 lated spines, those of the latter being quite simple and plain, 

 both on the pygidium and thorax. Similarly it is also dis- 

 tingTiished from A. Injstrix, Wy. Thomson, and A, Caractaci, 

 Salter ; in the first of these each pleura terminates in two 

 reflected spines, one passing under the other, and in the latter 

 * Quart. Jour. Geol. Soc, 1857, xiii., pp. 206-209, 



