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X. Description of a remarkable new Sea-urchin of the Genus Cidaris from Mauritius. 

 By F. Jeffrey Bell, M.A., Sec.R.M.S., Professor of Comparative Anatomy and 

 Zoology in Kings College. 



Received September 12tli, 1892, read Kovember 1st, 1892. 



[Plate XXXVIII.] 



A FEW months since the Trustees of the British Museum obtained from M. de 

 Eobillard, of Mauritius, another of those rarities in the collection of which he has 

 so much distinguished himself ^. The specimen is unique, and its general facies would 

 be so much altered by the removal of a fifth of the spinulation that I propose, on this 

 occasion, to limit myself to a description of the external appearance of the most 

 remarkable Cidaris it has ever been my good fortune to see. In the hope that further 

 examples might be discovered I have delayed, longer perhaps than I should, the publi- 

 cation of a notice of this extraordinary specimen. 



The primary spines are exceedingly long, some of them being more than 150 millim. 

 in length, or about three times the diameter of the test. They are, however, most 

 remarkable for being curved, slightly indeed, but yet distinctly curved in an upward 

 direction. The base of the spine is flattened on its lower side ; there are two sharp 

 edges, and the upper side is formed of two halves set at a wide angle to each other, 

 and ending in a distinct ridge. This ridge may be dentate and ornamented with a few 

 minute tubercles. At a distance of about 20 millim. from its base the upper ridge 

 disappears, and the spine becomes flat above as well as below. At about this point 

 most of the spines become completely altered in colour (in the dry specimen), for 

 while the basal part is creamy yellow, the rest of the large spine is of a reddish- 

 brown colour. In many, near the tip, there are a few bands of brown and pale yellow. 

 Where the brown colour begins a distinct striation also commences, and there are ten 

 strise on both the upper and the lower surfaces. The spine is widest at its base, and 

 as it narrows very regularly the whole has the form of a greatly elongated triangle. 

 Gradually and almost imperceptibly, the form of the spines in cross section alters, 

 and instead of being depressed and flattened it becomes almost regularly circular. 



The spines just described are arranged very regularly in pairs in each inter- 

 ambulacrum ; only one or two are more than 150 millim. long ; in each inter- 

 ambulacrum there are seven or eight primary spines, and the shortest are, as usual, 



1 I cannot let pass this opportunity of putting on record the regret with which all who are interested in 

 marine zoology have heard of the recent death of this distinguished collector. 



