16 ECHINODERMS OF THE CRAG. 



differing from Brissus carinatus, the fasciole of which makes two incurved flexures in 

 this region. It then winds closely round the end of the antero-lateral ambulacrum, and 

 ascends rapidly between it and the postero-lateral one. making a deep but wide flexure 

 somewhat truncated at its upper part ; it then curves down, following closely the bounds 

 of the postero-lateral ambulacrum, round which it winds and crosses the posterior 

 interambulacral space, with an arched curve not so deeply in-turned as the lateral curves 

 are. The anal extremity of the test is perpendicularly truncated (a character also distinctive 

 between this species and B. carinatus), the anus lenticular and large, and placed rather low, 

 the sub-anal or caudal fasciole is broadly subcordate, truncated below. The mouth is 

 transversely semicircular, with a slightly overhanging and prominent lip. The oral 

 ambulacra are subtriangular and radiating. The tubercles of the post-oral spinous space 

 are subequal and radiating. 



Length of specimen (presented by E. H. Bunbury, Esq., M.P.) in the Museum of 

 Practical Geology, 4 inches and -^ths. Breadth, 8 inches. Height, 2 inches and -^ths. 



This remarkable urchin is found in the Coralline Crag. Mr. Searles Wood and 

 Capt. Alexander have taken it, as well as Mr. Bunbury. It varies much in proportion, 

 but is distinctly identical with Scilla's species, which lives in the Mediterranean, and occurs 

 fossil in the miocene of Malta. I purposely omit all references to Lamarck, as there is 

 sad confusion about this Brissus and its allies. 



Genus — Amphidetus, Agassiz. 



Body cordate, tumid, with heterogeneous ambulacra converging to a genital disk, 

 which is dorsal and entire ; superior portion of the lateral ambulacra trumpet-shaped. 

 Anus terminal. A caudal and an intra-petal fasciole, the latter conspicuous on the back, 

 and shield-shaped. Eour genital pores. Mouth bilabiate, excentric, and placed anteriorly 

 on the ventral surface, in front of an escutcheon. Spines slender, curved, graduated; 

 no prominent and conspicuous primary tubercles on the dorsal surface. 



The living species of this genus are all from the North Atlantic, and its arms. The 

 fossil forms are all from the Upper Tertiaries. 



1. Amphidetus cordatus. Plate II, fig. 1. 



Echinus cordatus, Pennant. Brit. Zool., vol. iv, p. 69, figs. 34 and 75. 

 Spatangus pusillus, Leske. Page 230, tab. xxiv, figs, c, d, e, and tab. xxxviii, fig. 5. 



— arcuarius, Lamarck. An. sans Vert., vol. iii, p. 31, and 2d edit., vol. iii, 



p. 328. 



— — Be BlainviUe. Man. d'Actin., p. 201. 



— cordatus, Fleming. Brit. An., p. 480. 



