100 PSEUDODIADEMA 



primary tubercles, 12 — 14 in each row; interambulacral areas with two rows of primary 

 tubercles, 12 — 14 in a row, and two external rows of small secondary tubercles, extend- 

 ing from the peristome to the ambitus ; mouth-opening small, in a concave depression ; 

 tubercles of both areas nearly the same size. 



Dimensions. — Height six tenths of an inch ; transverse diameter one inch and one 



fifth. 



« 



Descripiion. — This Urchin is in general of medium size, with a sub-circular or pen- 

 tagonal test, convex above and flat below ; the ambulacral areas are large, slightly inflated, 

 and provided with two rows of small primary tubercles, 12 — 14 in a row, rather less than 

 those in the interambulacral areas, and separated by a double zigzag row of very small 

 granules (fig. 2 d), gradually diminishing in size from the ambitus to both poles; the 

 poriferous zones are subflexous and composed of pairs of small round holes placed in single 

 file throughout, crowded together near the peristome, and spread out above ; the inter-ambu- 

 lacral areas are twice the width of the ambulacral and furnished with two rows of primary 

 tubercles rather larger than those of the ambulacral areas ; they are very uniform in size 

 and gradually diminish from the ambitus to the poles ; between these rows and the 

 poriferous zones, and between the two rows themselves, a series of small tubercles, 6 — 8 

 in number, extends from the peristome to the ambitus, where they disappear ; these small 

 secondary tubercles fill up the intertubercular spaces at the base, and give the under sur- 

 face of the test a very ornamental appearance (fig. 2 h). There are fourteen plates in each 

 column of the inter-ambulacra, the primary tubercle occupying the centre of each ; the 

 areolas are circular and superficial, the bosses prominent and sharply crenulated, and the 

 mammillons large and deeply perforated (fig. 2 d) ; the surface of the plates is 

 sparsely covered with small granules which form circles around the areolas and are 

 scattered without order over the interspaces; the internal borders of the four upper- 

 most plates of both columns are nude (fig. 2 h) ; and the sutures distinctly visible 

 throughout their entire course. The ambulacral areas have fourteen plates in each column, 

 the tubercles are rather smaller than those in the inter-ambulacra, and the narrow areolas 

 are separated by a zigzag line of single granules (fig. 2/), which becomes double 

 (fig. 2 c?) near the ambitus. The miliary zone is large, nude, and depressed at the upper 

 part, granular towards the equator, and narrow and sinuous as it approaches the peristome 

 (fig. 2 c). 



The mouth-opening is small, and lodged in a deep depression, the basal portion of 

 the test being inflated around the peristome, which is decagonal, with nearly equal 

 lobes (fig. 2 3). 



The apical disc is absent in all the specimens I have examined ; the opening, however, 

 is large, a little elongated, subpentagonal, and angular (fig. 2«). 



AJfinities and Differences. — This Urchin is distinguished from its congeners by its in- 

 flated base, depressed upper surface, simple pores,numerous primary tubercles nearly uniform 

 in size and number in the columns of both areas, by its small unequal secondary tubercles, 



