90 ■ Dr. T. Wright on the Cassidulidse of the Oolites. 



umbrella figured by Agassiz, does not enable us satisfactorily to 

 compare that Urchin with our specimen, and leaves a doubt upon 

 our mind whether the Urchin known to Lamarck was identical 

 with this species. We have examples corresponding in form, 

 size, and comparative dimensions with Agassiz's figure, but the 

 absence of the sculpture of the plates in the Swiss specimen 

 leaves the question in doubt : through the kindness of Professor 

 Forbes we have compared our Urchins with the one sent by 

 Mr. Phillips from Yorkshire, and have proved their identity ; but 

 having had no opportunity of examining typical foreign speci- 

 mens, we have provisionally adopted Mr. Phillips^s specific name. 

 The ambulacra are one- fourth part the width of the interambu- 

 lacra ; they are prominent and convex, and furnished with four 

 rows of tubercles, the marginal rows extending from the mouth 

 to the apical disc, whilst the internal rows disappear near the 

 mouth and about half-way up the dorsal surface ; the interam- 

 bulacra at the circumference have from 18-20 rows of tubercles; 

 the number however gradually diminishes between the basal angle 

 and the mouth, and the circumference and the apical disc, so 

 that from two to four rows only extend from the mouth to the 

 vertex. The mammillated eminences on which the tubercles are 

 placed are encircled by smooth areolse, surrounded by small gra- 

 nules arranged round their circumference, which gives the in- 

 tertubercular surface of the plates a granular structure. The 

 tubercles are small, prominent and perforated, and the summits 

 of the mammae are crenulated as in the Cidaridse. These cha- 

 racters added to others, as the regularity of the arrangement of 

 the tubercles, and the dorsal position of the anal opening, lead 

 us to consider Pyg aster as a transition form connecting the 

 Cidaridse with the Cassidulidse. The base is concave, and the 

 mouth is central and situated in a considerable depression ; the 

 opening is about one-fifth the diameter of the test at the circum- 

 ference ; its margin is divided into ten nearly equal-sized lobes^ 

 the angles of which correspond to the divisional sutural lines be- 

 tween the ambulacra and the interambulacra. The anus is a large 

 oval opening in the upper half of the single interambulacrum, 

 forming a great gap in this part of the test, and constituting 

 one of the most important characters of the genus. In the living 

 animal this space was probably occupied by a membrane, but in 

 the fossil condition it has the appearance of the test having been 

 fractured and lost. The. apical disc is absent in all the specimens 

 of Pygaster that have passed through our hands. The pores are 

 set closely together in pairs throughout the entire range of the 

 avenues ; sometimes they have a slightly oblique disposition. The 

 spines adhering to the fine specimen before me are short, needle- 

 shaped, and delicately striated longitudinally. 



