140 THE FOSSIL ECHINOIDEA 



1. Ambltptgtjs subkotundus, Duncan & Sladen. Plate XXVI, Figs. 1-14. 



Test subdepressed ; marginal contour subcircular, the length exceeding the breadth 

 in a very slight but variable degree. The height is less than one half the breadth, and 

 proportional to the length as 0*41 : 1, the dorsal surface being regularly convex, and 

 the margins thick and well rounded. The anterior slope of the longitudinal profile is 

 straighter and more inclined than the posterior portion of the profile, which is more 

 arched and bombous, the test being thicker and more bombous in the area comprised 

 by the three posterior interradia than anteriorly; this character faintly presents its 

 greatest development along the median line of the odd posterior interradium, but the 

 whole test is so evenly rounded that no definite carination is produced. The outline of 

 the transverse profile is a regular, gentle, convex curve, with no tendency towards a 

 conoid form. The actinal surface is subconcave, with the peristome rather deeply 

 sunken and the surrounding portion of the test bombous, having a convex curve, which 

 forms a gently rounded continuation of the almost inflated margin. On the ventral 

 surface of some specimens there is a faint development of tumidity ih each of the 

 columns of plates of the interradia, the feature being most definite in the odd posterior 

 interradium, where it produces the appearance of a very faint wide groove, passing from 

 the periproct towards the margin. 



The apical disk is slightly excentric in front, being distant from the posterior 

 extremity -^qq of the entire length of the test. The generative pores, which are four 

 in number, are very large and slightly oval in shape, occupying nearly the whole of 

 their plate ; the anterior pair smaller and nearer together than the posterior pair. The 

 generative plates are all separated by prolongations of the central madreporiform 

 portion of the disk, which is in connexion with the right anterior generative plate, the 

 central portion being subcruciform in shape. The ocular plates are very small and 

 subpentagonal in shape, a straight base being apposed to the extension of the central 

 mass which separates the neighbouring generative plates. The two posterior ocular plates 

 do not touch, although no portion of the central plate extends beyond their base-line. 



The ambulacra are flush with the surface of the test, or with the poriferous zones 

 very faintly sunken ; they are moderately wide, and the petaloid portion extends quite 

 to the ambitus. The posterior pair are rather wider than the anterior pair, and these 

 again are rather wider than the odd anterior petal, which is the narrowest. 



The series of pairs of pores extends uninterruptedly from apex to peristome, the 

 inner pores forming a straight continuous line ; the two lines thus formed by the com- 

 panion zones of a petal diverge from one another gradually as they proceed from the 

 apex, and converge again slightly on the ventral surface as they approach the peristome. 

 On the upper portion of the ambulacra (the petaloid part) the poriferous zones are wide, 

 the pores being wide apart and conjugate, having the inner pore round and the outer 

 one elongate and connected by a furrow. The intervening dissepiment is rather broad, 

 and is ornamented with a single line of small miliary granules. The outline formed by 

 the petal is lanceolate, and the widest part is at about two thirds of the distance from 

 the apex to the margin. The widest part of the poriferous zone is nearer the apex, 

 nearly midway between the margin and the apex ; at this point the poriferous zone is 



