192 Dr. T. Wright on Fossil Echinoderms 



Description. — Much confusion has arisen as to the identity of 

 this Urchin, occasioned probably by the circumstance of Scilla 

 having figured only the base of the test, and neglected to give 

 either its profile or the dorsal surface. In M. Agassiz's ' Pro- 

 dromus' it was entered as Micraster Goldfussii, but has been 

 omitted from Agassiz and Desor's ' Catalogue raisonne/ M. 

 Desmoulins identifies it with the Spatangus crassissimus of 

 Defrance, but on referring to the origmal description * of that 

 species, we find that Defrance's species came from " la craie 

 chloritee near Havre," and as no species of Urchin known to us 

 passed from the secondary to the tertiary epoch, we must reject 

 the high authority of this most accurate naturalist, and consider 

 the Urchin before us as distinct from S. crassissimus. The test 

 inclines to an oblong form (1 c) ; it is higher behind than before, 

 and declines more rapidly from the disc to the anterior border than 

 from the disc to the anus. The ambulacral areas lie in deep 

 depressions; the single ambulacrum (le) is the longest, and forms 

 an inconsiderable anteal sulcus ; the antero-laterals are Y^i^^ of an 

 inch in length, and form an angle of 44° ; the number of the 

 pores (1 /) in the zones is fifteen and twenty: the postero-laterals 

 are scarcely one-half the length of the antero-laterals ; they form 

 an angle of 56° ; the number of pores in them is respectively seven 

 and ten : the peripetal fasciole is broad, but feebly marked, and 

 closely embraces the bases of the areas : four sulci (1 «), nearly 

 destitute of tubercles, mark the course of the ambulacra from 

 the side of the fasciole to the mouth : the lateral interambulacra 

 are rather inflated, and marked by five or six angular elevations 

 (1 c) of the test : the single interambulacrum is elevated on the 

 dorsum (1 a), squarely truncated on the posterior border, and 

 convex beneath : the small oval anus is situated very high up, 

 about y^jjths of an inch from the disc ; the sternal portion of the 

 interambulacrum (1 b) is convex and prominent, and is covered 

 with eiose-set tubercles, arranged in lines, proceeding like radii 

 from a central point of the base : the mouth has a large pro- 

 jecting under-lip. The upper part of the shell is covered with 

 small close-set uncrenulate tubercles (1 «?), which are larger 

 and irregularly disposed on the front and base : the apical disc 

 is small and nearly central : there are only two genital h©les 

 at the apices of the lateral ambulacra : the five eye-holes are 

 very small. Whether one pair of genital holes may be a generic 

 character of Hemiaster, we have not the means at present of de- ' 

 termining, as most of our specimens of this group have the disc 

 concealed ; but about the beautiful H. Scillce now before us there 

 can be no question. 



* Diet. Sc. Nat. torn. 50. p. 96. 



