68 Dr. T. Wright on Fossil Echinoderms 
contained in the British Museum, Geological Museum, Jermyn 
Street, and Bristol Museum; that from the Ile de Caprée is in 
our cabinet. 
History.—First figured by Scilla, in 1670. The list of 
synonyms prefixed tu 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. tom. iii. p. 290, 2nd 
ed. ; Deslongchamps, Encycl. Méthod. t. ii. p. 200 ; Defrance, 
Dict. Sc. Nat. t. ix. p. 450; Blainville, Man. Act. p. 216; 
Grateloup, Foss. Ours. p.40; Agassiz and Desor, Cat. raisonné, 
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 6,4, inches, breadth 
6,4; inches, height at the centre 1,8; inch, thickness of the 
margin about 3th of an inch. 
Description.—This magnificent Urchin was figured by Seilla. 
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 Echinide 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- 
