J 26 On Fossil Echinoderms from the Island of Malta. 



inclining to oblong, being sligbtly compressed on the sides ; 

 ambulacral areas narrow, even with the interambulacra ; pori- 

 ferous zones very narrow; the inner and outer pores nearly 

 equal in size, and extending through three-fourths of the 

 areas ; base concave ; mouth nearly central, with large oral 

 lobes ; anus large, transversely oblong and inframarginal. 



Dimensions. — Antero-posterior diameter 6 inches, transverse 

 diameter 5^^^ inches, height 3 inches. 



Description. — This noble Urchin has been mistaken for C. 

 conoideus, Lamk., as in form, size, and some of its general cha- 

 racters it resembles that type species ; but the eye of the prac- 

 tised zoophytologist detects, in the structure and narrowness of 

 the poriferous zones, an organic character sufficient to enable 

 him to separate it from that species. The general outline of the 

 base is round, inclining to oval from the gentle compression of 

 the sides thereof; the dorsal surface is much elevated and highly 

 convex, and the vertex is situated in front of the centre of the 

 dome ; the ambulacral areas are nearly one-fourth the width of 

 the interambulacral areas at the border, and are level with them ; 

 they are nearly of a uniform width throughout, becoming lan- 

 ceolate at their upper iifth : the poriferous avenues are very nar- 

 row, and extend tlu'ce-fourths of the distance between the apex 

 and the border : the pores in the avenues are only about one line 

 apart, and are united by short slits directed obliquely upwards 

 and inwards at nearly equal spaces apart from the base to the 

 apex ; the pores in the outer and inner series in each zone are 

 about the same size throughout : the narrowness of the poriferous 

 avenues forms a very important character in this species, by 

 which it is distinguished from an allied form, C. conoideus. The 

 interambulacra are of a triangular form ; the antero-lateral are the 

 smallest, and the postero-lateral and single interarabulacrum of 

 about the same size are the largest : the apical disc is small, and 

 situated nearer the anterior than the posterior border, which 

 occasions a slight difference between the angle of inclination of 

 the anterior and posterior sides of the test : the centre of the 

 disc is occupied by a prominent button-like spongy madrepo- 

 riform body, around which the genital and ocular plates are 

 arranged, but their lines of suture can only be distinguished in 

 weathered specimens, or by removing the superficial layer of the 

 plates. The entire surface of both areas is covered with tuber- 

 cles, veiy much alike both as regards size, form, and irregularity 

 of arrangement ; the only parts exempt from tubercles are the 

 interporiferous septas of the avenues : the base is concave : the 

 mouth is situated in the centi'e of the disc, and is surrounded 

 by five large prominent lobes : the ambulacra form pctaloid 

 depressions between the oral lobes, in which a number of pores 



