100 Dr. T. Wright on the Cassidulidae of the Oolites. 



to be merely the margins of the depressions in the centre of which 

 the tubercles are placed. The apical disc is situated in the ver- 

 tex and is nearly central ; the plates of which it was formed are 

 absent in all the specimens I have seen ; the single and the an- 

 terior pair of ambulacra converge around its anterior border, and 

 the posterior pair terminate about two lines from the latter at 

 the posterior and external side of the disc. The single interam- 

 bulacrum is somewhat wider and longer than the others ; the 

 anal opening is situated in its dorsal part in a deep valley with 

 parallel vertical sides covered with tubercles ; about the middle of 

 the opening, the valley expands and forms a concave depression 

 in its lower half; the basal portion of the area is slightly pro- 

 duced, and forms a lip-shaped process, which gives a considerable 

 convexity and prominence to the basal angle of this area. 



Affinities and differences. — H. agariciformis differs so widely 

 from its congeners in our Oolites that it cannot be mistaken for 

 either of them; its shield-shape and depressed vertex distin- 

 guish it from H. gibberulus, and its dimensions, circular form, 

 and rounded posterior border serve as diagnostic characters be- 

 tween it and H. caudatus. From Pygaster, with which form it has 

 been erroneously grouped, it is distinguished by the greater 

 number and microscopic character of the tubercles in Hyboclypus, 

 the deep anal valley with its parallel vertical walls, and the ex- 

 centric five-lobed mouth ; whereas in Pygaster the tubercles are 

 fewer and larger, the anal opening forms a large space without 

 a valley and parallel walls, and the mouth is central, and has 

 its margin divided into ten lobes like Holectypus and all the 

 Cidaridse. 



Locality and stratigraphical range. — This species is tolerably 

 abundant in the lower ferruginous beds, " the Pea-grit " of Leck- 

 hampton, Crickley, Birdlip, Coopers, and Cleeve Hills : as far as 

 we at present know, it is not found in the middle division of the 

 Oolites. 



History. — This species has been elaborately figured for the 

 4th decade of Professor Forbes' s 'Memoirs of the Geological 

 Survey ' from the specimens now before me, and will be described 

 in detail in that work, to which I beg to refer for more ample 

 particulars. 



Hyboclypus caudatus, Wright, n. sp PI. III. fig. 2a-e. 



Test oblong, much depressed ; single interambulacrum produced 

 into a caudal prolongation; mouth very excentric, near the 

 anterior border ; apical disc and vertex slightly excentric, an- 

 terior border blunt, posterior border truncated. 



Height /^ths of an inch, antero-posterior diameter 1 inch and 



