188 Dr. T. Wright on Fossil Echinoderms 



developed, from their close approximation to those of the oppo- 

 site area : the apical disc is small ; the four genital holes are 

 large, the anterior pair being more closely approximated than the 

 posterior pair ; it is situated nearer the anterior than the posterior 

 border and lies in a confluent depression, in which the apices of all 

 the areas freely converge. The single ambulacrum is rather longer, 

 but not so wide as the anterior pair ; its lateral row of single holes, 

 with their accompanying tubercles, are small and indistinctly 

 seen, and it forms an inconsiderable anteal sulcus : the posterior 

 border is squarely and obliquely truncated, and in its upper part 

 near the dorsal surface is the large anal opening : the base is 

 rather convex ; the sternal portion of the single interambu- 

 lacrum is slightly prominent, aud ornamented with a few rows 

 of rather larger tubercles disposed in zigzag lines : the basal 

 tracks of the ambulacral areas are entirely naked, and v;hcre 

 they terminate around the mouth five petaloid poriferous radii are 

 observed. The mouth, of moderate size, is in the anterior third ; 

 the peripetal fasciole is narrow and indistinct ; the subanal 

 fasciole is much broader, and remote from the anus, but the test 

 is unfortunately broken in this region ; the tubercles are nearly 

 all of the same size, but a few larger ones occupy the sides, front, 

 and base. 



Affinities and differences. — The flatness of the dorsal surface, 

 the deep depressions made by the petaloid portion of the am- 

 bulacral areas, and the double crescent formed by them, i-eadily 

 distinguish B. crescenticus from its congeners. So few fossil 

 species of this genus have been figured or described, that we can 

 only compare it with the other forms obtained from the same bed, 

 from both of which it differs in many well-marked characters. 



Locality and stratigraphical positio7i. — It was collected from 

 bed No. 4, the calcareous sandstone at Malta, where it is rare. 



Genus Hemiaster (Desor, 1847). 



Urchins with a high and much inflated test ; ambulacral sum- 

 mit nearly central ; the petaloid portions of the ambulacral areas 

 situated in depressions more or less deep ; the antero-lateral are 

 in general much longer than the postero-lateral pair; the peripetal 

 fasciole only surrounding in an angular manner the ambulacral 

 star. This genus differs from Micraster in all the species having 

 a more inflated body with a peripetal fasciole ; from Brissopsis in 

 having the postero-lateral ambidacra in general much shorter, 

 and the anterior and posterior pairs more unequal in length, 

 and likewise in having no subanal fasciole, A very few species 

 are found in the tertiaries, the majority belonging to the cre- 

 taceous rocks. 



