ACIDASPIS GRAY^B. 115 



of which the posterior one is the stronger. Surface of pleurae minutely pitted, and 

 borders ornamented with single row of minute tubercles. At extremity of each 

 pleura are three spines ; (1 ) the bordering ridges unite and are produced, into a 

 long median straight or gently curved spine directed backwards, and armed with 

 minute denticles on its anterior and posterior edges. In front of this median spine 

 there projects (2) a very small blunt spine, and behind the former is likewise (3) a 

 short spine, stouter than the anterior one. 



Pygidinm very broadly semicircular, about three times as wide as long (not 

 including spines), flattened, horizontally extended, with narrow, smooth, raised 

 lateral border bearing spines, and narrower raised border on straight anterior 

 edge. Axis convex, conical, prominent, about one fourth the width of the 

 pygidium, and composed of two segments and a half ring on front end for articula- 

 tion. The first segment is a regular prominent ring, tuberculate and defined 

 behind by a strong furrow; the second segment (which may be of composite origin) 

 is longer and larger, subquadrate or bluntly subtriangular in shape and tuberculate, 

 with an obscure ring on its anterior part defined by a pair of pits behind it. 

 Lateral lobes flattened ; surface reticulate and traversed by a curved ridge arising 

 from the first axial segment and ending on the border at the base of the sixth pair 

 of spines. Border of pygidium narrow, smooth, raised, and furnished with eight 

 pairs of spines, radiating, straight, and of equal length, those of the posterior pair 

 being parallel. The spines are armed with minute lateral denticles, alternate on 

 opposite sides of the same spine and sub-alternate on opposite sides of adjacent 

 spines. 



Remarks. — The three species Acidaspis grayae, A. hystrix, and J l<iht</<' are 

 found associated in the same bed and in the same locality, and are undoubtedly 

 closely allied. In fact I have much doubt whether A. grayx is really separable 

 from A. lalage, but the material at present available is not sufficient to decide this 

 point. The type of A. grayse is in Mrs. Gray's collection, and is in a poor state of 

 preservation, but it consists of an entire individual, which is satisfactory, as Ave 

 have generally to deal with detached heads and pygidia. In this type-specimen 

 the head is badly preserved, and the spines on the margin of the pygidium are 

 broken off short, but the other specimens of the pygidium have the spines attached 

 and the denticles preserved. As Nicholson and Etheridge remark, the distinction 

 from A. lalage seems to consist chiefly in the denticulated spines, for the head- 

 shield is too poorly preserved to make a minute comparison. As far, however, as 

 its characters can be made out, it has similar proportions, the lobation of the 

 glabella agrees, the occipital lobes are developed in the same manner, and the 

 occipital ring has the same relative dimensions, but it is apparently devoid of the pair 

 of long diverging spines, so conspicuous in A. lalage. Etheridge was wrong in the 

 statement that the pygidium had only fifteen spines, for the bases of sixteen can 

 lie counted on his type-specimen figured in the 'Proceedings of the Royal Physical 



