184 PSEUDODIADEMA. 



The poriferous zones are very narrow ; the pores are contiguous, and unigeminal 

 throughout (fig. 2 d), except at the wide inter-tubercular spaces around the mouth, where 

 they fall into triple oblique pairs (fig. 2 b); in the upper third of their course, the zones 

 curve slightly inwards, and the pores are set very closely together in this region. There 

 are five pairs of pores opposite each inter-ambulacral plate, and nearly a like number 

 opposite the ambulacral plates (fig. 2 d). 



The inter-ambulacral areas are four tenths of an inch in width, and only one fourth 

 wider than the ambulacral areas. They retain their breadth very uniformly throughout, and 

 are occupied by two rows of primary tubercles, of which there are ten in each row ; the 

 bosses are large and prominent ; those at the equator occupying nearly the entire surface 

 of the plates (fig. 2 d) ; their summits are sharply crenulated, and the spinigerous tubercle 

 is widely perforated ; from the peristome to the equator (fig. 2 b, d), or for about two 

 thirds the length of the area, there is a row of small secondary tubercles between the 

 zones and the primaries, which are raised on mammillated bosses, with crenulated summits 

 (fig. 2 d) ; the miliary zone is narrow, and composed only of two rows of granules, which 

 descend down the tract of the zigzag central suture (fig. 2 d), there being five granules on 

 each plate (fig. 2 d). 



The tubercles of both areas stand prominently out from the surface of the test, in 

 consequence of the size of the mammillary bosses, which are unusually conical in this 

 species ; from the mouth to the equator the tubercles of both areas are nearly alike in size 

 and number (fig. 2 b, c), but on the upper part of the test those of the ambulacral areas 

 become much smaller and more numerous (fig. 2 a) than those of the inter-ambulacral 

 areas. 



The mouth opening is wide, about eleven twentieths of an inch (fig. 2 b) ; the peris- 

 tome is decagonal, and unequally lobed, the ambulacral being twice as large as the inter- 

 ambulacral lobes. Since PI. VIII was printed, I have obtained a slab with the test and 

 spines in situ thereon, and which is figured in PI. XII, fig. 8. The primary spines 

 (fig. 8 a, b) are nine tenths of an inch in length ; the head is well defined by a prominent 

 milled ring ; the stem, which is cylindrical, tapers uniformly from the ring to the point, 

 which is rather blunt, and the surface is covered with fine longitudinal lines ; the secondary 

 spines (fig. 8 c) are nearly three twentieths of an inch in length, and are miniature repre- 

 sentatives of the primaries. 



The disc opening is wide (PL VIII, fig. 2 d) ; its form cannot be described, as the 

 plates around its margin are more or less fractured. The discal elements are lost in all 

 the specimens I have yet seen. 



Affinities and differences. — The great width of the ambulacral areas, and the uniformity 

 in the size and number of the primary tubercles at the base and equator of the test 

 (fig. 2 b, c, d), render the diagnosis between this species and its congeners easy and 

 decisive. 



