310 



COLLYRITES 



As this urcliin presents very variable proportions, I subjoin a table, by Professor 

 Forbes, showing the dimensions of eight specimens from the inferior Oolite of Dorsetshire, 

 measured in inches and twelfths. 





A 



B 



c 



D 



E 



F 



G 



H 



Length 



ItV 



It'. 



oii 



0-I«o 



OH 



Ot% 



Oif 



m 



Breadth 



1 



hh 



m 



Of-j 



Oh 



0-^ 



Oit 



m 



Thickness . 



0^ 



0-h 



OtV 



0^ 



0-A 



Ot^. 



Ot\ 



OA 



Description. — The preceding table shows how much the general outline of this curious 

 urchin varies in different individuals, so much so, indeed, that out of the varieties of 

 Collyntes rin(/ens, no less than three other species, C. Eudesii, Agass., C. Jc/assizii, 

 d'Orb., C. suhringens, M'Coy, have been described and proposed as distinct species. 

 Having carefully examined and compared upwards of one hundred specimens of this 

 urchin, I can confidently state that the orbicular, sub-pentagonal, and oblong varieties met 

 with in the Inferior Oolites of Dorsetshire, are all referable to one and the same species, 

 the extreme forms in different individuals being blended together by numerous intermediate 

 gradations of structure. 



The dorsal surface is uniformly smooth and convex, and more or less depressed, 

 it is elevated posteriorly, and declines gently anteriorly, the vertex in general is situated 

 nearer the anterior than the posterior border (fig. 3 c) ; the sides are tumid, the 

 anterior border is flattened, and the posterior is produced and truncated (fig. 3 c) ; the 

 antero-lateral is in general narrower than the postero-lateral region, and the prominence of 

 the inter-ambulacral spaces in some individuals produces the sub-pentagonal varieties. 

 The base is very unequal from the convexity of the inter-ambulacra, which form five 

 nodulose eminences around the mouth ; the posterior single area in particular is extremely 

 prominent, gibbous (fig. 3 (5, c), and much deflected ; its posterior border is truncated and 

 channelled to form the anal valley (fig. 3 c, d), which is bounded by two ridges, com- 

 mencing at the apices of the posterior pair of ambulacra, and passing downwards and 

 outwards towards the base, where they may be traced on the summit of the single area as 

 far as the mouth (fig. 3 h). The anal opening has a pyriform shape, with its apex directed 

 upwards ; it is situated in the upper part of the valley, nearly on a level with the dorsal 

 surface, and inmiediately below the ambulacral arch (fig. 3 d, c). 



The ambulacral areas are all complete, at the dorsal surface they are on a level 

 with the inter-ambulacral plates, but at the base, they lie in depressions. They are 

 of unequal width, the single area being the narrowest, the posterior pair the widest, 

 and the anterior pair of intermediate breadth ; the single area is straight, and the anterior 



