170 Dr. T. AYright on new Species of Echinodermata 



Diadema Davidsoni, Wright. PL XII. fig, 2, a-e. 



Test depressed, circular; tubercles elevated upon prominent 

 uiammillary eminences ; pores in a single file throughout ; a 

 few small secondary tubercles in the interambulacra ; the pri- 

 mary ambulacral tubercles nearly as large as those of the 

 interambulacra. 



Height g^yths of an inch, transverse diameter 1 inch and /oths. 



Description. — This beautiful Urchin has a regular circular 

 test, not at all inclined to the pentagonal form of many of its 

 oolitic congeners. The ambulacral areas are three-fourths the 

 width of the interambulacral areas, and are nearly of a uniform 

 width throughout, tapering slightly and gracefully inwards to- 

 wards their superior third ; the contraction assumes the form 

 of a gentle curve slightly inclined towards the centre. The 

 double row of tubercles gradually increases in size from the 

 mouth to the equator, where three pairs are about the same size ; 

 from this point upwards they gradually decrease, and terminate 

 in two pairs of minute rudimentary tubercles at the disc. A 

 single row of granules, arranged in a zigzag form, separates the 

 primary tubercles from each other, a larger granule marking each 

 of the angles. There is no granulation or other sculpture be- 

 tween the mammillary eminences of the tubercles and the pori- 

 riferous avenues. There are from twelve to thirteen pairs of 

 tubercles in each area. The interambulacral areas are nearly 

 y'^^ths of an inch in width, and ^^th wider than the ambulacral 

 areas; they retain their width uniformly throughout, and are 

 occupied by two rows of primary tubercles, nine to tea in a row, 

 the mammillary eminences of which are large and prominent, 

 and separated from each other by two rows of small granules 

 which extend only a short distance beyond the equator ; the 

 remaining space between their termination and the disc being 

 destitute of sculpture, where likewise the areas are slightly de- 

 pressed; and a single row of granules rises on the external side 

 of the tubercles, with here and there a secondary tubercle towards 

 the basal portion of the test. 



The poriferous avenues are very narrow : the pedal pores are 

 arranged in a single file, only three or four additional pairs being 

 introduced in the increased spaces around the circumference of 

 the mouth. The tubercles of both areas are of moderate size, but 

 exceedingly prominent, in consequence' of being elevated upon 

 large mammillary eminences, the apices of which are deeply cre- 

 nulatcd. The tubercles of the ambulacral areas at the equator 

 are not much less than those of the interambulacral areas, but 

 u{)on the upper surface of the test they become much smaller 

 and more numerous. 



