48 CIDARIS. 



The apical disc is absent in all the specimens I have examined. It was of considerable 

 size ; the diameter of the opening in specimen b being -ffths of an inch. 



The spines are of three kinds, the primary, the secondary, and the tertiary. (PL II, 

 fig. 2 d, e,f.) The primary spines or radioles are very elegant bodies, and as they are often 

 preserved in the Corallian stage, when the test to which they belonged is not discovered, 

 a knowledge of them is of stratigraphical importance to the student of the Jurassic rocks. 

 The concave articulating cavity has a deep pit in its centre (fig. 2 e) for the insertion of a 

 ligament, and the rim of this acetabulum is surrounded with a circle of moniliform crenu- 

 lations ; the head is small, and is surrounded by a narrow ring, nearly smooth, and covered 

 only with a microscopic milling of longitudinal lines ; the ring is midway between the rim 

 of the acetabulum and the point where the head articulates with the neck (fig. 2 e) ; the 

 neck is short, and is covered with fine longitudinal lines ; it soon expands to form the body 

 of the spine, the thickest part of which is just beyond the neck, from whence the stem 

 gradually tapers to the apex, which is always more or less truncated (fig. 2 d) ; the stem 

 is covered with small granulations, very uniform in size, and regularly disposed in longi- 

 tudinal rows, forming from twenty to thirty lines of tubercles on the body of each spine, 

 the number of the rows varying in different spines ; the tubercles of the adjoining rows 

 alternate, so as to produce a quincuncial arrangement ; and the tubercles of each series are 

 connected together by a calcareous filament which passes from one tubercle to another ; the 

 tubercles are all inclined towards the apex of the spine, and many of them terminate in 

 short, prickly processes, which have their points directed forwards ; the surface of the 

 spine between the rows of granules is covered with numerous longitudinal lines : at the 

 summit of the spines the granules coalesce, forming so many plates, which expand, and 

 produce a radiated or star-like disc at the truncated extremity thereof. Some of the spines 

 attain the length of two inches and three quarters. The secondary and tertiary spines 

 are short and spatulate (fig. 2/), and their surface is covered with fine longitudinal lines. 



Affinities and differences. — In the form and structure of the test in general, Cidaris 

 jiorigemma resembles Cidaris Blumenbachii, margi?iata, coro?iata, and monilifera, but it 

 is distinguished from all these forms by its greater height, and a consequent increase in 

 the number of plates in the tubercular columns ; Cidaris marginata, coronata, and 

 monilifera having five plates in each column, whilst Cidaris Jiorigemma has from six to 

 seven. In the plates on the upper part of the test of these species, the miliary zone is 

 much wider, and the circles of areolar granules are likewise separated by many rows of 

 miliary granules, which are altogether wanting in Cidaris jiorigemma. The ambulacral 

 areas in Cidaris marginata, coronata, and monilifera have four distinct rows of small, 

 nearly equal-sized granules, whilst in Cidaris jiorigemma the two marginal rows alone are 

 well developed throughout. 



Cidaris jiorigemma is distinguished from Cidaris Fowleri by the narrowness of its 

 ambulacral areas, and by the size and prominence of the marginal granules ; the 



