366 MOIKA ATROPOS. 



The odd anterior ambulacrum is divided, by the closing up of the slightly 

 sunken edge of the abactinal part of the edge of the anterior groove, into two 

 connected cavities. When seen from above, the projecting lips form two 

 triangular spaces (PL. XXIII. f. 5), closely covered by small tubercles, to 

 the anterior edge of which the peripetalous fasciole extends, and then passes 

 on to the edge of the lateral ambulacra. The anterior lateral ambulacra 

 are much longer than the posterior. The vertex is situated slightly behind 

 the apical system; the posterior extremity is truncated vertically, making a 

 sharp angle with the actinal plastron, which has, immediately below the sub- 

 anal fasciole, a small, fully developed keel (PL XXIII. f. J t ). The anal sys- 

 tem is flush with the test, small, elliptical, edged by eight large plates sur- 

 rounding the smaller interior ones. The lateral fasciole is narrow, becoming 

 slightly broader under the anal system ; the part of fasciole adjoining the 

 deep groove is broad; the connecting parts between the petals are narrow. 

 Seen in profile, the anterior edge is gradually rounded from the vertex to the 

 ambitus, the same curve of the test extending to the mouth (PL XXIII f. .'). 



Fragments of this species were dredged from a depth of 80 fathoms. 

 Girard has attempted to separate specimens from Texas, of slightly more 

 elongate outline, as a distinct species. The color of M. atropos when alive 

 is yellowish. The spines, where more thickly clustered, are brownish ; they 

 are short except where they cover the sunken ambulacra, which are entirely 

 hidden by the longer spines meeting from both sides. On the lower surface, the 

 interambulacral plastron is covered by long spines, wdiich, as they wear out 

 at the extremity, become spatula-shaped. On the side of the ambitus, and 

 on the upper lateral part of the posterior ambulacra, the spines attain a 

 greater length, especially towards the mouth, where they are most closely 

 crowded together. 



Littoral to 80 fathoms. 



