FROM THE GREAT OOLITE. 237 



notches, with reflected edges, indent the bases of the inter-ambulacra, and extend as far as 

 the areolas of the second or third tubercles. 



The primary spines are finely preserved in situ in fig. 4 k j they are variable in size in 

 the same and in different specimens, arid are proportionally shorter in young than in old 

 urchins ; sometimes they are three times the length of the diameter of the body (fig. 4 b), 

 and are sometimes nearly three inches and a half in length ; the head is conical, with a 

 truncated extremity marked by deep crenulations (fig. 4 m), the ring is prominent, and the 

 milling is angular and sharp ; the stem (fig. 4 b) swells out beyond the ring, it is more or less 

 irregularly sub-angular, with the angles rounded ; a transverse section of one of the spines 

 exhibits an irregularly elliptical figure ; sometimes the spine tapers to a conical point, or the 

 distal end terminates in a bifid or trifid extremity. The secondary spines articulating with 

 the ambulacral tubercles (fig. 4 n) are short, about three tenths of an inch in length, they 

 are round, and taper gently from the ring to the point, their surface being covered with 

 fine longitudinal lines (fig. 4 n). 



The jaws are preserved in one or two specimens (fig. 4 g) ; the lantern is strong 

 (fig. 4 h), the teeth conical, and in its general appearance the dental apparatus closely 

 resembles that of an Echinus. 



Affinities and differences. — This urchin very much resembles a Hemicidaris ; in fact, 

 Acrosalenia and Hemicidaris have so many characters in common, which are always well 

 preserved, and so few that are diagnostic, and which for the most part are either broken or 

 absent, that it requires considerable practice, when the apical disc is wanting, to deter- 

 mine the genus ; the absence of semi-tubercles at the base of the ambulacra and the 

 magnitude and pentagonal form of the discal opening serve as good guides to the 

 genus. Acrosalenia hemicidaroides is distinguished from A. Lycettii by having a larger 

 and more spheroidal test, larger bosses on the sides, and smaller ambulacral tubercles ; 

 the apical disc is larger, and the sur-anal plate is composed of a greater number of pieces. 

 Acrosalenia hemicidaroides is distinguished from A. pustulata in having regular rows of 

 secondary tubercles in the ambulacral areas, which are larger and more fully developed ; 

 the miliary zone is narrower, with only two rows of granules ; the primary tubercles are 

 more developed at the upper surface, and the apical disc is likewise larger. 



The same characters distinguish it from A. Wiltonii ; but besides those already enu- 

 merated, that species has four rows of fine granules in the miliary zone, smaller tubercles, 

 suddenly diminishing at the upper surface, and a much smaller mouth opening. The 

 complex character of the sur-anal plate, the size of the test and of its primary tubercles, 

 readily separate it from A. spinosa. 



Locality and Sir atigraphical position. — The finest specimens of this urchin with their 

 spines attached (fig. 4 k) have been collected by Mr. William Buy, from the Forest 

 Marble, near Malmesbury, in a thin vein of clay, which, according to that acute and 



