BEITISH FOSSILS. 



a 



of similar tubercles appear, but in loose order and not placed in 

 regular line with the rest. Altogether about twenty rows (some- 

 times more) may be counted at or about the margin of an interam- 

 bulacrum. On the under surface (fig. 3.), the transverse rows are 

 very distinct, and the difference between the central and the 

 exterior portions is particularly manifest. As the plates become 

 square and less transverse near the mouth, the rows become more 

 distant and irregular, till there are again (in the angles of the 

 notches) only the two primary rows left. The tubercles there are 

 of larger size than those represented in our fig, 8. 



"The base is concave, and the mouth is central and placed in 

 a considerable depression. It is of moderate size, about one- 

 fifth the diameter of the test " in half-grown specimens, but less 

 in the adult. It is ten angled, the notches equally deep, but 

 in pairs nearer together beneath the interambulacrum than at the 

 avenues. 



The plates of the disk are absent in all our specimens. They 

 appear, however, to have been depressed. From the figures given 

 by Agassiz of the closely allied Swiss species, the disk must have 

 been oblong, the two posterior lateral ovarian apertures being 

 placed rather wide apart, and the hinder one absent. The anal 

 aperture is very large, and in our specimens being confounded with 

 the space left by the disk plates, its shape cannot be very well 

 ascertained. It occupies in adult specimens full half the length of 

 the odd interambulacrum, and appears to be oblong-oval rather 

 than pyriform. Its sides are incurved, but the depression in which 

 it lies does not extend farther* than the anus itself, although the 

 tumid ridge on each side of it continues for some way down. In 

 young specimens the anal opening is very much smaller, and propor- 

 tionally narrow. " The spines adhering to this specimen are short, 

 needle shaped, and delicatel}^ striated."" (Wright, 1. c.) 



Variations. — We have figured Dr. Wright's very fine specimen, 

 which is unusually large and has the tubercles more conspicuous 

 than common, nor has it any convexity in the ambulacra beyond 

 what arises from their forming the angles of the pentagon. Other 

 specimens from the same locality agree with it in nearly all respects, 

 but have the tubercles of less size and more numerous, tv^^enty-four 

 in a row across the interambulacra, and as many as six at the 



* This is one cf the many characters in which our species differs from the had figure 

 of Galerites umbrella, given in the Encyc. Methodique, and which has been quoted again 

 and again by Agassiz as a synonym of his Pi/gaster umbrella, a species closely allied to 

 the one here described. 



