306 INFULASTER. 



The dorsal surface is very high anteriorly (fig. 1 c), and the walls of the anteal sulcus 

 rise nearly perpendicularly to the highest part of the body (fig. 1 d, e) immediately in 

 front of the apical disc (fig. 1 a). From the point of its greatest elevation the upper 

 surface gradually declines nearly in a straight line (fig. 1 c, i), forming a long sub-carinated 

 ridge, which extends to the upper part of the posterior border (fig. 1 d), at about half the 

 heio-ht of the test at the vertex. On each side of the ridge the upper surface slopes 

 away rapidly (fig. 1 d), but the sides are rounded, and, in the antero-lateral regions, 

 tumid (fig. 1 e). The posterior border forms a triangular space (fig. 1 c, d, i) ; which is 

 obliquely truncated, and has the periprocte opening in its upper angle surrounded by a 

 circle of tubercles two rows deep (fig. 1 d). 



The under surface is convex in the antero-posterior, and transverse diameters, so that 

 the under surface of the body is gently rounded in all directions ; on the cheeks at the 

 base a few primary tubercles are observed (fig. 1 ^), and on the plastron another grouping 

 of like tubercles is placed. 



The single ambulacrum is lodged in the deep anteal sulcus, but the poriferous zones 

 are scarcely discernible in good specimens. The lower half of the depression is rounded 

 and covered with minute granules, among whicli no indications of pores can be seen with 

 a hand magnifier; in the upper third they are small, indistinct, and closely set together, and 

 are best seen where the uppermost portion of the area curves backward to meet the apical 

 disc (fig. 1 a, c). Here we see a few pairs of small round pores, arranged in a vertical series, 

 and the single ambulacrum articulating with the disc a httle in advance of the antero-lateral 

 ambulacra ; these are placed very far forward and extend over the most tumid part of the 

 sides of the test (fig. 1 c and i). The pores are very small (fig. 1 a). Forbes counted on 

 a large specimen thirty pairs of pores in these rows between the apex and the margin ; 

 in my figured specimens only twenty can be identified. The avenues or poriferous zones 

 are at the surface of the test, and the pairs of pores of the outer rows are a little 

 wider than the inner one. The postero-lateral ambulacra are even less conspicuous 

 (fig. 1 c,d) ; they diverge greatly from the antero-laterals, and form together a very acute 

 angle. Their uppermost portion consisting of closely-set pores. On their posterior 

 portions the pairs of pores become very indistinct. 



The apical disc is small, narrow, and elongated (fig. 1 /^). There are four perforated 

 ovarial plates in the middle, and five very small perforated ocular plates at the summits 

 of the areas ; all the parts of the disc are very solid and firmly joined into the other 

 elements of the test. 



The mouth is situated near the anterior border (fig. 1 It) at the point wliere the 

 anteal sulcus terminates, it therefore lies in a depression. It is a small opening with a 

 round or transversely oblong peristome ; the anterior lip is smooth, and the posterior 

 more developed. 



The vent is situated in the upper third of the truncated posterior border (fig. 1 d). 

 The periprocte is vertically oval and of large size (fig. Id); its upper half is surmounted 



