FROM THE GREAT OOLITE. 



235 



Test hemispherical, more or less depressed, sometimes elevated ; ambulacral areas 

 narrow, undulated, with two rows of small, perforated tubercles, fourteen to sixteen in 

 each row, which alternate on the borders of the area, and gradually diminish in size from 

 the base to the apex ; inter-ambulacral areas with large prominent tubercles on the sides, 

 and small tubercles near the disc; areolas sub-confluent; miliary zone narrow, with two 

 rows of granules ; apical disc very large ; sur-anal plate composed of six elements ; mouth 

 large, decagonal ; peristome deeply notched ; primary spines twice or more in length the 

 diameter of the test; stem sub-angular, tapering, or slightly compressed ; extremity some- 

 times bifid or trifid. 



Dimensions. — This species varies so much in size and figure, that I have selected four 

 specimens on account of their differences, the comparative dimensions of which are shown 

 in the following table : 



Acrosalenia hemicidaroides. Wright. 



Largest form. 



Elevated form. 



Depressed form. 



Common small form. 



Inch. 



Inch. 



Inch. 



Inch. 



Transverse diameter of test 

 Height of test 

 Diameter of mouth opening 

 Length of apical disc 



1t% 

 0^ 



Wo 

 0^ 



Wo 

 0^0 





Description. — This is the most common and best preserved of all our fossil sea-urchins ; 

 it has long been known as a Cornbrash species, but was neither named, figured, nor 

 described, until I gave its history, with figures and details, in my ' Memoir on the Cidaridae 

 of the Oolites ;' since then it has been figured and described by Professor Forbes in the 

 fourth Decade of the ' Memoirs of the Geological Survey ;' and by M. Desor, in tabl. XX, 

 figs. 19 — 23, of his valuable ' Synopsis des Echinides Fossiles.^ This species exhibits much 

 variation in size and figure, but its diagnostic characters are preserved with remarkable 

 uniformity throughout these different phases of form and magnitude. 



The test is sometimes elevated and globular, like a Hemicidaris ; indeed, the large 

 forms, the dimensions of which are given in the first column of the table of measurements, 

 are commonly so named ; the absence of semi-tubercles at the base of the ambulacra, 

 and the large size of the elongated discal opening, are the only characters by which they 

 can be distinguished from that genus. In the more common form (fig. 4 c), the body 

 is spheroidal and depressed on the upper surface ; and, when the flattening is excessive, it 

 produces the depressed form of the third column. 



The ambulacral areas are narrow and moderately prominent (fig. 4 a) ; they are 

 nearly of a uniform width, gradually expanding in the lower half, and tapering in 



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