19i Dr. T. Wright on the Cassidulida? uf the Oolites. 



&ths, transverse diameter 1 inch andf^tk. The great majority 

 of the specimens average as follows : — 



Height /yths of an inch, antcro-posterior diameter |§ths of an 

 inch, transverse diameter £§ths of an inch. 



Description. — The test of this elegant little species is covered 

 with tuhercles so minute, that without the assistance of a good 

 lens, the observer might suppose that it was altogether destitute 

 of sculpture ; the single and anterior pair of ambulacra are 

 straight and very short in consequence of the excentricity of the 

 mouth and vertex, and terminate at the anterior border of the 

 apical disc ; the posterior pair are one-seventh longer, and curve 

 upwards, inwards, and forwards on the dorsal surface, termi- 

 nating by the anal valley at a short distance from the posterior 

 border of the disc. The pores are placed closely together on 

 the dorsal surface, but are situated at wider distances apart at 

 the base. 



The interambulacra are of unequal width ; the anterior pair are 

 the shortest and narrowest, the posterior pair the widest, and the 

 single area the longest, which is likewise considerably produced 

 into a lip-like process, which curves gently downwards and is 

 abruptly truncated posteriorly. The anal furrow is deep with 

 vertical parallel walls, which gradually expand into two ridges, 

 corresponding with the truncated borders of the lip-like process. 

 The anterior border is blunt, with a slight depression in the 

 middle formed by the single area; the base is concave and 

 slightly undulated; the mouth is situated near the anterior 

 border, and is a simple pentagonal opening without lobes ; the 

 tubercles in this region are somewhat larger, but they are fewer 

 in number and arranged with much irregularity on the plates. 

 The apical disc is unfortunately broken in all the specimens we 

 have examined ; the space which it occupied is however very 

 small. 



Affinities and differences. — H. caudatus differs from its conge- 

 ners by its oblong form, posterior lip-like process formed by the 

 single interambulacral area and its depressed and excentrical 

 vertex ; it is distinguished from H. gibberulus by the absence of 

 the anterior central ridge characterizing that species, and from 

 H. agariciformis it is distinctly separated by its oblong form and 

 excentric mouth and anus. 



Locality and stratigraphical range. — This is not a common 

 Urchin ; it is found occasionally in the lower and upper beds of 

 the Inferior Oolite at Leckhampton, Crickley, and Birdlip Hills, 

 and it occurs occasionally in the planking beds of the Great 

 Oolite at Minchinhampton. The specimens from the latter lo- 

 cality are in general small and not well preserved ; the individual 

 whieh we figure is the largest we haye seen. 



