-)40 ROTULA. 



they are iiioi-c or less longer and less clavate. The spines of the median 

 lateral interambulacral spaces poinl towards the actinostome, while those of 

 the ambulacra and posterior interainhulacrum point towards the edge ; the 

 reverse is the case in Mellita The coloration was of a uniform light reddish- 

 yellow. 



We owe to Mr. W. II. Dall the first discovery of the locality of this inter- 

 esting genus; he collected specimens in Japan, and in the Bulletin of the 

 Museum the species was excluded from the geographical lists of the Pacific 

 shores of America. Dr. Martens had also collected it previously in Japan, 

 but supposed it to he a tertiary fossil, till his attention was called to it by 

 the papers of Yerrill and Trosehcl. 



ROTULA. 



Rotula Klein, 1734, Nat Disp. Echin. 



Test exceedingly flat ; outline circular; anal extremity digitate or deeply 

 lohed. Actinostome small, central ; anus nearer the mouth than the edge. 

 Ambulacral petals small, with a tendency to diverge ; pores extend as far as 

 the edge. On the actinal surface the ambulacral furrows are studded with 

 small pores ; furrows only branching twice, thus forming one branch for 

 each digitation of the margin. The structure of the abactinal system is 

 unique among Clypeastroids ; the whole system is raised, forming a promi- 

 nent star-shaped body, the points and ridges corresponding to the ambulacral 

 system. The four genital openings are placed in the centre of the sides of 

 the abactinal pentagon ; the ocular plates occupying the apex. The reverse 

 is the case in all other Scutellidae. The madreporic body is comparatively 

 small. The edge is supported by clusters of radiating partitions, extending 

 but a very short distance towards the centre. The median pari of the interior 

 surface of the lower part of the test is smooth ; the marginal notches cor- 

 respond mainly to the interambulacral spaces, there being but one cut in the 

 median ambulacra, except in the odd ambulacrum. The jaws are short, with 

 calcareous lamellae on the two sides ; those of the upper edge much larger 



