Dr. T. Wright on the Cidaridse of the Oolites. 261 



The interambulacral area? are nearly three times the width of the 

 ambulacral areas, and support two rows of from six to eight large 

 perforated tubercles raised upon crenulated mammillary emi- 

 nences ; the base is flat, the mouth large, decagonal and notched, 

 and the margin reflexed. The apices of the notches point to the 

 centres of the columns of the interambulacral plates. 



Acrosalenia hemicidaroides, Wright, n. s. PL XI. fig. 1 a, b, c, d. 



Test hemispherical, considerably depressed ; ambulacral areas with 

 two ranges of from fourteen to sixteen small perforated tuber- 

 cles, gradually decreasing in size from the base to the apex ; 

 interambulacral areas with two ranges of primary tubercles, 

 eight in each range. The supra-anal plate is composed of 

 several elements ; the anus is situated before and to the left 

 side ; the surface of the ovarial, ocular, and supra-anal plates 

 is studded with small granulations; primary spines long, 

 tapering, smooth and slightly compressed ; secondary spines 

 small and needle-shaped ; mouth large and decagonal ; margin 

 reflexed. 



Height ygths of an inch, transverse diameter 1 inch and ^th. 

 One large specimen measures 1 inch and T 3 oths in diameter, but 

 the proportional height cannot be ascertained, as its base is 

 crushed. 



Description. — Test spheroidal, depressed; ambulacral areas 

 slightly sinuous, nearly uniform in breadth, tapering towards 

 both poles, and supporting two rows of secondary mammillated 

 perforated tubercles, which are largest at the inferior third of the 

 area, diminishing in size as they approach the mouth and the 

 anus. The tubercles, from fourteen to sixteen in number in each 

 row, are situated alternately on the margins of the area ; a zigzag 

 line of granulations, with lateral branches passing down the cen- 

 tre of the area, separates the tubercles from each other. The 

 poriferous avenues consist of about forty-five pairs of pores set 

 obliquely in a single file. The interambulacral areas are three 

 times the breadth of the ambulacral ; each area is composed of 

 two columns. There are eight plates in each column, and each 

 plate supports a large smooth mammillated eminence surmounted 

 by a tubercle, which occupies the greater part of the plate ; it is 

 of a conical form, and is encircled by a concave smooth areola. 

 The summits of the mammas are sculptured on their margins with 

 eleven crenulations, in the centre of which a deeply perforated 

 tubercle rises, with a rather depressed articular surface. In 

 some specimens the areolas of the mammas are confluent, in 

 others they are separated by a row of small granules. The ex- 

 ternal and internal margins of the plates are furnished with rows 



