BRITISH FOSSILS. 



Decade IV. Plate II. 



ACROSALENIA HEMICIDAROIDES. 



[Genus ACROSALENIA. Agassiz. (Sub-kingdom Radiata. Class Ecliinodermata. 

 Order Ecliinoidea. Eamily Echinidse.) Body spheroidal, usually depressed ; ambulacra! 

 and interambulacral segments developed, the former bearing two rows of small secondary 

 tubercles, the latter two rows of unequal large primaries ; tubercles perforate, and placed 

 on crenulated bosses. Anus excentric, included within the apical disk, which is formed of 

 five genital and five ocular plates, with one or more supplementary central plates. Ambu- 

 lacral avenues with the pairs of pores falling into single file above and on the sides, and 

 becoming distinctly three-ranked near the mouth.] 



Reference. Acrosalenia hemicidaroides. Wright, in Annals'] and 

 Mag. of Nat. Hist., 2d ser. vol. viii. p. 161. pi. xi. fig. 1. (1851.) 



Diagnosis. A. ambulacris angiistis, tubercuJis parvis approximatis 

 alternatis ornatis ; inter amhulacris tuberculis primariis conspicuis, ad 

 latera magnis, superne parvis, areolis subcoiijiaentibus ; area centrali 

 an gustissima, granulata, granidis biseriatis. 



This, one of the most beautiful and best preserved of British 

 Oolitic Sea-XJrchins, has been for some years a well known orna- 

 ment of collections, but received no specific appellation until it was 

 described and figured in 1851 by Dr. Wright of Cheltenham, in his 

 excellent memoir on the Cidaridce of the Oolites. 



The body is spheroidal, depressed above, usually considerably so, 

 but not always. At first glance its most striking fea^tures are 

 the large size and fewness of the interambulacral tubercles and 

 their bosses, whilst those on the ambulacra are very small and but 

 slightly spaced out. The interambulacral segments are centro- 

 laterally thrice the breadth of the ambulacral ones, and are some- 

 what prominent. The latter are narrow, gently and gradually 

 widening in the upper portion of their lower half, and exliibit a 

 slightly undulating contour ; they bear two rows of very small, 

 nearly equal, and prominent secondary tubercles, each row con- 

 sisting of (in large specimens) about 16, all of them perforated and 

 placed upon bosses, each of which exhibits about 10 crenulations. 

 [lY. ii.] 4 C 



