362 METALIA PECTORALIS. 



the lateral ambulacra is less marked in young specimens. The anal system is 

 placed in a depression in the obliquely truncated posterior extremity of the 

 test; it is pear-shaped, the small end turned towards the abactinal pole, 

 often rising above the edge of the test. The plates covering the anal sys- 

 tem are irregular, nearly of uniform size ; the plates covering the membrane 

 immediately surrounding the anal opening are minute, occupying the greater 

 part of the actinal portion of the anal system ; in the smallest specimen 

 measured the posterior extremity is truncated vertically, there being but a 

 slightly marked anal depression, which becomes afterwards so prominent with 

 advancing age as to form in large specimens cheeks on each side of this de- 

 pression at the angle of the subanal fasciole and the anal branch. The whole 

 upper surface of the test is covered by minute tubercles, forming a regular 

 granulation below the peripetalous fasciole to the edge of the test. Above 

 the fasciole the tubercles are slightly larger and more distant, especially 

 when forming the scrobicular circle round the primary tubercles. The geni- 

 tal openings are circular, diverging posteriorly, with prominent comma-shaped 

 madreporic body extending into the posterior interambulacrum. 



The actinal plastron is elongate, narrow, with a marked keel at the posterior 

 extremity extending to the base of the broad, heart-shaped subanal plastron, 

 has four pores, completely surrounded by a broad fasciole, lobed internally. The 

 anal fasciole is independent of the subanal, being separated from it by an 

 extremely narrow ridge of minute tubercles; the horizontal part is broad; 

 the upper branches narrow, and extend to the edge of the test. The tuber- 

 cles of the actinal surface are largest outside of the ambulacra] rows, gradually 

 decreasing towards the edge of the test ; the tubercles of the actinal plastron are 

 considerably smaller, and more closely packed. In large specimens the suture 

 of the plates on the upper part of the test are slightly sunken, the plates them- 

 selves sloping from a vertex or ridge towards the edges, — a feature not 

 prominent in younger specimens, thus forming ridges dividing the interam- 

 bulacrum into distinct fields. The fine figure of this species given by Seba 

 is one of the best which can be given, and will serve, as many of his figures 

 do now, to readily identify many species of Echini. The mouth is com- 

 paratively narrow transversety, distinctly rounded anteriorly, with a straight, 

 raised posterior edge. The actinal membrane is covered by polygonal plates, 

 closely crowded, except the anterior row of plates, which is the largest, de- 

 creasing gradually to the posterior edge. 



The identity of the pliocene and post-pliocene species cited in the synonymy 



