﻿404 



PYGURUS 



Description. — The beautiful specimen figured in PL XXXIX is the only one of 

 this form I have seen. The upper surface, sides, and outer part of the base, are in 

 fine preservation, but the greater portion of the under side is concealed by the matrix. 



The test is thin, and has a sub-orbicular circumference ; it is rounded before, and 

 slightly produced behind, the difference between the length and width being only one 

 quarter of an inch. 



The dorsal ambulacra are narrow, only slightly expanded in their upper half, and 

 terminating in sharp, lanceolate apices around the disc (fig. 1 a) ; they have six rows of 

 tubercles, disposed alternately on the plates, so that they form double oblique rows, with 

 three tubercles in each (fig. 1 d). 



The poriferous zones, of moderate width, are petalloid five sixths of the distance 

 between the border and disc ; as in all other Pyguri, the holes of the inner row are 

 nearly round, those of the outer row are oblique or nearly transverse slits, which about 

 equal in length the width of one half of the area (fig. 1 c) ; between each slit-like aperture 

 there is a partition of the test, on the surface of which a series of ten granules are very 

 regularly arranged (fig. 1 d) in a single row ; at the borders the pores lie close together, 

 in single pairs, but they are wider asunder at the base. 



The inter-ambulacral areas are of unequal width ; the anterior pair are about one 

 sixth narrower than the posterior pair ; they are formed of long, narrow plates, which are 

 bent to an obtuse angle in the middle ; along the line of these angles the surface of the 

 test is slightly elevated, producing in the middle of each area a triangular elevation, the 

 base of which is at the border, and the apex towards the disc (fig. 1 a) ; the margin, in 

 like manner, exhibits a fulness corresponding with the bases of these elevations. 



The apical disc is absent, and the space for its reception is small in comparison with 

 the size of the test. 



The tubercles are beautifully preserved in this species (fig. 1, c, d) ; on the upper 

 surface each plate carries four horizontal rows, which are arranged in zigzag order 

 above one another; the tubercles are all perforated, and crenulated, and raised on small 

 mam miliary eminences; the areolas which encircle them are wide and well defined ; a circle 

 of miliary granules surrounds the areolas, and other granules fill up all the intermediate 

 spaces ; the granules are surrounded by narrow areolas, which impart a highly sculptured 

 character to the surface of the test. 



The anal valley lies so near the posterior border that it produces an emargination 

 thereof (fig. 1 a) ; when viewed from behind, the vent is seen quite in the border of the 

 rostrated portion of the single inter-ambulacrum (fig. 1 b). 



The tubercles at the border and base are much larger and more prominent than those 

 on the upper surface ; and the areolas present a regular hexagonal disposition around the 

 margin and at the base. 



I have only seen one specimen of this urchin, embedded on a portion of Coralline 

 Oolite limestone ; the test is very thin, and has been fractured, the joint having been 



