294 HYBOCLYPUS. 



with an acute and sinuous margin arising from the convexity of the inter-ambulacra 

 (fig. J c) ; the base is flat, or moderately concave, and is slightly undulated by the 

 depressions formed by the ambulacra, and the convexities by the inter-ambulacra. In 

 general the antero-posterior equals the transverse diameter, but sometimes the transverse 

 exceeds the antero-posterior diameter (fig. la, 6). 



The ambulacral areas are very narrow and of unequal width ; the anterior single area 

 is the narrowest; the anterolaterals are a little broader than the odd one, and narrower 

 than the postero-laterals (fig. 1 a) ; the three anterior areas converge around the front of 

 the apical disc, whilst the postero-laterals curve sinuously up to its posterior border 

 (fig. 1 a) ; each area has four or six rows of small tubercles, so arranged on the plates that 

 they form oblique rows, which meet in the median line, and branch upwards and 

 outwards, thereby forming V-shaped figures (fig. Id). 



The poriferous zones are narrow; on the dorsal surface the pores are unigeminal, 

 and placed close together (fig. 1 d); at the base, in consequence of the plates being broader, 

 they are wider apart, and form slightly oblique ranks of threes ; whilst nearer the mouth 

 they lie in very oblique trigeminal ranks (fig. 1 e) ; the oral portions of the ambulacra form 

 a radiate rosette around the mouth. 



The inter- ambulacral areas are very wide, but of unequal width ; on an average they 

 are eight times as broad as the ambulacrals (fig. 1 a, b). In one specimen the anterior 

 pair measure at the circumference one inch and three tenths, the posterior pair one inch 

 and eleventh twentieths, and the single area is one tenth of an inch wider than the 

 posterior pair ; the margin of all the areas is convex, except the posterior single inter- 

 ambulacrum, which is slightly truncated. The dorsal surfaces of the anterior and posterior 

 inter-ambulacral and of all the ambulacral areas are gently convex ; but the single inter- 

 ambulacrum, which is somewhat wider and longer than the others, has a deep valley with 

 parallel vertical sides in its dorsal half ; these gradually decline and expand into a concave 

 depression at the lower half (fig. 1 a, c) . 



The anal aperture opens into the upper part of this valley immediately below the 

 apical disc ; the basal portion of the area is slightly produced and truncated, and forms a 

 lip-shaped process, which imparts a considerable prominence and convexity to it (fig. 1 c). 



The base is concave and undulated (fig. 1 b, c), the ambulacra lie in nearly straight, 

 depressed valleys, whilst the inter-ambulacra form gentle convex eminences at the circum- 

 ference; the mouth opening is small and excentral, being nearer the anterior border; 

 when well exposed, the peristome is seen to be unequally decagonal, the ambulacral being 

 larger than the inter-ambulacral lobes (fig. 1 b). 



The apical disc is small and central ; the margin of the opening is notched with four 

 inter-ambulacral and five ambulacral notches, the former corresponding to the external 

 angle of the ovarial plates, the latter to the margins of the oculars (fig. 1 a) ; the disc 

 appears to have been lodged in a depression ; but in none of the hundreds of specimens 

 which I have examined have I detected any of its elements. 



