82 HEMICIDAEIS. 



with faintly marked crenulations ; the areolas are rather wide, and only slightly grooved, 

 so that the tubercles project prominently and abruptly from the surface of the test. The 

 margin of the areola is encircled by a row of thirteen granules (fig. 5 c, d), rather larger 

 than those which cover the rest of the inter-tubercular surface of the plates. The miliary 

 granules are close-set, and disposed without much regularity on the surface of the plates. 

 The apical disc (fig. 5 b) is of moderate size, and slightly prominent ; the five ovarial 

 plates are large, and of a heptagonal form ; the ocular plates are small and heart-shaped, 

 and the surface of both is covered with a close-set, delicate granulation ; the anal opening 

 is nearly central, and circular ; the base is flat ; the mouth opening is large and widely 

 decagonal, from the great size of the ambulacral lobes, and the comparative smallness of 

 the inter-ambulacral. The spines are as yet unknown. 



Affinities and differences. — This remarkable little urchin is so entirely different from 

 its congeners, that it is impossible to mistake it for any other of the group to which it 

 belongs. The presence of semi-tubercles at the base of the ambulacral areas only, and of 

 granules on the sides of these spaces, associate it with Hemicidaris diademata, but the 

 small number of the primary tubercles on the inter-ambulacral areas, added to the great 

 distance at which they are placed apart, serve to distinguish it from the young of that 

 species ; in fact, these characters alone are perfectly diagnostic of Hemicidaris minor among 

 all other forms of Hemicidaris. 



Locality and StratigrapMcal position. — It was first found in the " Grand Oolite " of 

 Langrune, Calvados, whence the beautiful specimen before me was obtained, and kindly 

 sent by Professor Deslongchamps, of Caen. I take the present opportunity of recording 

 my grateful acknowledgements to that eminent natm'alist for his kindness and courtesy, 

 not only in contributing specimens to my cabinet for comparison and reference, but like- 

 wise for communicating many rare species of Oolitic Echinidae, which served as the types 

 of several of M. Agassiz's new species, and which specimens have been of much service to 

 me in clearing up doubts as to the identity of some other English forms. 



Hemicidaris minor was collected in this country by W. Walton, Esq., from the Great 

 Oolite of Hampton, near Bath. I have never found this species in the Great Oolite of 

 Minchinhampton, nor have I seen it in any collection of fossils from that locality. 



History. — Eirst named by M. Agassiz from specimens in Professor Deslongchamps 

 and M. Michelin's collections ; afterwards described as Acrosalenia rarispina by Professor 

 M'Coy, from specimens in the Cambridge Museum, which came from Mr. Walton's series, 

 collected near Bath. It was figured and described in detail, for the first time, in my 

 contributions to the Palaeontology of Gloucestershire, published in the ' Annals and 

 Magazine of Natural History^ for 1854. 



