FROM THE INFERIOR OOLITE. 277 



line (fig. 1 d). Two of these rows, the fifth on each side from the centro-sutural line, repre- 

 sent the primary tubercles, and extend from the peristome to the disc ; the other rows, 

 as the plates become narrower, disappear at various points on the sides, so that, whilst at 

 the margin of the specimen (fig. 1 d) there are twenty rows of tubercles, in the vicinity 

 of the apical disc there are only two rows (fig. 1 a). The tubercles are nearly of the same 

 size, and form very regular horizontal and vertical rows on both areas ; those of the 

 primary rows are larger in the upper surface, and all the basal tubercles are so likewise. 

 The tubercles, which are perforated, are raised on small bosses, with smooth summits 

 (fig. \ e,f) ; their base is encircled by narrow, sunken, ring-like areolas, which are much 

 more developed around the ventral than on the dorsal tubercles ; the circumference 

 of the areolas is surrounded by a circle of small granules, and the intervening portion 

 of the plates is covered with a like-sized miliary granulation (fig. 1 d, e,f). 



The opening for the apical disc is very large, half an inch in diameter, but in only one 

 of the hundreds of specimens of this species which I have examined has a vestige 

 of the plates remained. These consist of four ovarial, and three ocular ; the ovarial plates 

 are small, and dove-tail with the angular incisions in the discal opening ; the spongy 

 madreporiform body is large, extending inwards and backwards, and the ovarial holes are 

 perforated near the apex ; the ocular plates are very small, situated at the summit of the 

 ambulacral areas, and the eye-holes are perforated near their margin ; as the centre of the 

 disc is absent; the form of its posterior boundary is therefore unknown. 



The anal aperture is a very large, oblong-oval opening, which occupies nearly the 

 upper half of the single inter-ambulacrum (fig. 1 a). Its shape forms an important 

 diagnostic specific character, and ought to be carefully noted in making determinations 

 of the species. The borders of the opening are incurved, and the vent appears to 

 have lain in a depression ; at the point where the plates incline towards the lateral parts 

 of the vent, two tumid ridges extend downwards to the posterior border, having a 

 slight concave depression between them (fig. 1 a, c). In young specimens, the anal 

 aperture is proportionately smaller. 



The base is concave, and the mouth central, and placed in a considerable depression. 

 It is of moderate size, about one fifth the diameter of the test ; the peristome is nearly 

 equally decagonal, the ambulacral being larger than the inter-ambulacral lobes ; the 

 notches are wide, and the margin is everted (fig. 1 b). Although I have searched diligently 

 for the teeth, I have never yet seen a vestige of one, although there cannot be a doubt 

 that the Piasters possessed jaws like those of Holectypus depressus, Leske. 



The spines adhering to the fine specimen I have figured are short and needle-shaped, 

 and delicately striated longitudinally. 



Affinities and differences— Pygaster semisulcatus resembles in many points Piaster 

 umbrella, Agassiz, but is distinguished from that urchin by the following characters: 

 The tubercles, especially those on the upper surface, are disposed in much more regular 



