DOROCIDARIS BLAKEI. 



11 



system (PI. II. Figs. 1. 2) which would undoubtedly lead to their being mn- 

 sidered specifically distinct. Set, if the isolated huge fan-shaped radioles 

 bad alone been dredged, radioles | PI. II. Figs. 7-10) nearly identical in shape 

 with those of the Jurassic Rhabdocidaris remus Des. (Phyllacanthus Br.) few 

 palaeontologists would have hesitated to refer them to thai genus. 



A comparison of the differences in the tesl of this species and of I), papil- 

 laia shows that the coronal plates (PI. II. Fig. I) have a comparatively larger 

 and more elliptical sorobicular area, surrounded l>\ a -in^ie row f | ar g er 

 secondary tubercles; the tubercle and boss are much smaller, and the tuber- 

 culated spaces of the median interambulacral area are also narrower. In a 

 specimen measuring .'17 mm. there are (i-17 primary interambulacral plates, 

 while in a smaller specimen of D. papillaia, measuring 35 nun., there are 7-8 

 primary interambulacral plates. In the ambulacra! areas the poriferous zone 

 is nearly as broad as the median ambulacral spaees (PI. II. Fit;-. 1). while in 

 I), papillata it is narrower; the secondary ambulacral tubercles are also 

 smaller than in that species. It also differs from D. papillata in having a 

 smaller anal system, but this is covered by a larger number of plates inside 

 the outer row (PI. II. Fig. 2) than in D. papillaia. The ornamentation of the 

 radioles is the same on the long fusiform, or cylindrical, or the more or less 

 fan-shaped radioles of the test (PL IL Figs. 3-10); these figures show well the 

 gradual passage from a long, sharp-pointed, cylindrical radiole (PI. II. Fig. 3) 

 to a huge fan-shaped radiole (Fie-. 10), through the successive stages of EVs. 

 4-1). in which (he radioles become little by little more flattened and spread- 

 ing at the extremity. As the radioles become more fan-shaped, the rows of 

 spinules are gradually changed into minute serration-, spreading more and 

 more, and becoming somewhat less prominent towards the extremity, rows of 

 smaller serrations being intercalated as the extremity of the radioles becomes 

 more and more fan-shaped. The broad end of the fan-shaped radioles is 

 sometimes slightly concave. 



In the first specimens of this species, dredged by Captain Sigsbee, there 

 were no radioles showing the intermediate stages here figured between 

 the Ion-;, sharp, cylindrical radiole- of Plate II. Figs. 3, I. and such fan- 

 shaped ones as are figured in Plate II. Figs. !>. 10. Enough has been 

 shown from the examination of this species to show how little we are as 

 yet able to determine among the Cidarid.e the value of either generic or 

 specific characters. Before we can hope to make tic much needed accu- 

 rate revision of this family we need a large mass ol material, especially 



