from the Island of Malta. 115 



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. alius, exhibits much deviation from, what 

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

 study of the Echinidce 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- 

 bulacral areas. From the base of the areas to the circumference 

 the margin is thin and expanded, and in this respect resembles 

 a Scutella much more than a Clypeaster. The interambulacra 

 between the poriferous avenues form convex elevations, which give 

 a stellate character to the central dome, all the more conspicuous 

 as it rises abruptly from the thin expanded margin, which is 

 almost destitute of any elevation. The tubercles are larger on 

 the basal than on the dorsal surface. In only one of the specimens 

 before us is the inferior surface exposed. The base is flat. The 

 pentagonal mouth is much smaller than in C. altus. In a speci- 

 men before us, measuring Ai-J-^ inches in length, the mouth- 

 opening is /oths of an inch in diameter; the oral lobes curve 

 inwards and form the interspaces thereof. Acute narrow am- 

 bulacral grooves pass outwards to the circumference. 



Affinities and differences. — The thin and broadly expanded 

 border of C. marginatus y with its short ambulacra, and central 

 dome rising suddenly from the middle of the test, form a group 

 of characters which enable us readily to distinguish this species 

 from its congeners, with one exception, C. Tarbellianus. The 

 excellent figures of this Urchin, given by Grateloup in his able 

 Memoir*, we have compared with two forms of C. marginatus 

 from Malta, and we confess our inability to distinguish the 

 differences between them and the author's type-figure. Agassiz 



* Mem. sur les Ours. Foss. de Dax, p. 40. pi. 1. fig. 5-6. 



8* 



