68 Dr. T. Wright on Fossil Echinoderms 



contained in the British Museum, Geological Museum, Jermyn 

 Street, and Bristol Museum ; that from the lie de Capree is in 

 our cabinet. 



History. First figured by Scilla, in 1670. The list of 

 synonyms prefixed to this article exhibits the various epochs in 

 its history. In none of the works we have consulted is any 

 detailed description of the species given. 



Clypeaster marginatus, Lam. 



SYN. Scilla, Corp. Mar. tab. 11. fig. inferior. 



Clypeaster marginatus, Lam. An. sans Vert. torn. Hi. p. 290, 2nd 

 ed. ; Deslongchamps, Encycl. Method, t. ii. p. 200 ; Defrance, 

 Diet. Sc. Nat. t. ix. p. 450 ; Blainville, Man. Act. p. 216 ; 

 Grateloup, Foss. Ours, p.40; Agassiz and Desor, Cat. raisonne, 

 t. vii. p. 131 ; Desmoulins, Etudes des Echinides, no. 12. p. 218. 



Test large, depressed, subpentagonal ; margin thin, broad, and 

 expanded ; outline of the border undulated ; ambulacral areas 

 short, oval, and convex, rising abruptly from the thin border, 

 and forming a dome-shaped elevation in the centre of the 

 dorsal surface; base flat; mouth-opening small and penta- 

 gonal, with five simple sulci extending from the angles 

 thereof to the margin ; anus small, round, and submarginal. 



Dimensions. Antero-posterior diameter 6 T 7 <y inches, breadth 

 6-^ inches, height at the centre l T 8 y inch, thickness of the 

 margin about ^th of an inch. 



Description. This magnificent Urchin was figured by Scilla. 

 The specimens before us agree very well with his drawing, 

 although the foreshortening of the dorsal surface does not give a 

 sufficient elevation to the ambulacral dome. The expansion of 

 the margin, and thinness thereof, make a marked distinction 

 between this and other cognate forms. The figures given by 

 Grateloup of his C. Tarbellianus so exactly resemble the large 

 specimen before us, belonging to the Bristol Museum, that we 

 no longer doubt that species being a variety of C. marginatus. 

 This species, like C. altus, exhibits much deviation from what 

 may be considered to be its type form. A long and attentive 

 study of the Echinida has shown us, that such difference of 

 outline is the rule, and not the exception, in the group ; and 

 that specific characters must be drawn from organic structure, 

 and not merely from outline, if we wish our species to have a 

 permanent place in the register of Nature's forms. The ambu- 

 lacral areas are gracefully petaloid, rounded at the base and 

 tapering towards the apex ; they are convex and prominent, and 

 extend about half-way between the vertex and the border, the 

 test rising into a dome-shaped elevation in the region of the am- 



