CAKE-URCHIN. 1 79 



Echinus placenta, Gmelin, 3195. 



Echinarachnius placenta, Fleming, Brit. An. p. 479. Gray. Agassiz, Prod. 

 Scutella placenta, Lamarck, Anim. sans Vert. 1 Edit. vol. III. p. 11 ; 



2 Edit. vol. III. p. 283. Desmoulins, Echin. p. 228* 

 Echi?iodiscus placenta, Blainville, Man. d'Actin. p. 218. 



Through the kindness of Professor Jameson I am en- 

 abled to figure and describe the only British example of 

 this remarkable Sea-Urchin, one of the many additions he 

 made to the British Fauna when examining the zoology of 

 the North many years ago. He obtained it from very deep 

 water off the island of Foulah, one of the Zetland group. 

 It is usually recorded in zoological manuals as an inhabi- 

 tant of the southern seas ; whether it has so wide a range 

 is questionable, but it is undoubtedly an inhabitant of the 

 North, as the specimen I describe testifies, and in Mr. 

 LyelFs collection there are several exactly similar from the 

 coast of Canada. The Foulah specimen measures two 

 inches and three-fourths across. 



The Cake-Urchin is allied to several genera, presenting 

 very singular and eccentric forms, — forms which may be 

 regarded as linking the Sea-Urchins with the true Star- 

 fishes, and the study of which, therefore, presents many 

 points of great interest. Even the arrangement of the 

 parts in the species before us is very Starfish-like, especially 

 on the under surface. In form it is nearly circular, and 

 very much depressed, the centre only being somewhat 

 prominent. The apex is occupied by a pentangular tuber- 

 cle or button, similar to those seen on the apices of Spa- 

 tangi, and which prominences may be regarded as of the 

 same nature with the madreporiform tubercles of the 

 Echinus and the Starfishes. From this button radiate 

 five pairs of short ambulacra. In the arrangement of these 

 ambulacra there is a remarkable generic peculiarity, which 

 was kindly pointed out to me by Professor Agassiz. It 



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