96 HEMICIDARIS. 



The difference between this species and Ilemicidaris Bravenderi has been already 

 pointed out in the article on that urchin, and its affinities with Hemicidaris Purbeckensis 

 and Ilemicidaris Davidsoni will hereafter be discussed in the description of those species. 



Locality and Stratigraphical position. — This species is sometimes very common, and 

 found in fine preservation in the Coralline Oolite at Calne, Wilts. It is collected in the 

 Coral Rag at Weymouth, and in the same rock near Faringdon, in Berkshire ; from the 

 Coralline Oolite of Malton, in Yorkshire, it is likewise rarely obtained. I have before me 

 specimens from all these localities. From Hildenley, near Malton, some very fine shells 

 have been collected, but the Yorkshire specimens in general are not well preserved. 



History. — This, in all probability, is the urchin which was figured by Martin Lister* 

 and by Plott,t and has been always a much admired and abundant Echinite. Until 

 separated by Dr. Fleming under the name Hemicidaris intermedia, it was confounded 

 with Hemicidaris crenularis of continental authors ; and I have good reasons for believing 

 that it is often mistaken for that species by foreign palaeontologists, as I have received speci- 

 mens ticketed Hemicidaris crenularis which undoubtedly belong to the English species. In 

 Hemicidaris intermedia the base is always much more inflated than the same region of the 

 test in Hemicidaris crenularis, which tapers more, and gives the shell a more globular 

 and elegant form. Compare, for example, our figures with the excellent drawings of 

 Hemicidaris crenularis, given in Goldfuss's ' Petrefacta Germanise.' 



D. Species from the Portland Oolite. = 16th Mage Portlandien, d'Orbigny. 



Hemicidaris Davidsoni, Wright, nov. species. PL IV, fig. 2 a, b, c, d. 



Test subglobose ; ambulacral areas slightly undulated, upper half with two rows of 

 minute marginal tubercles, lower half occupied by an irregular row of large semi-tubercles ; 

 primary tubercles of the inter-ambulacral areas well developed, and gradually diminishing 

 in size from the equator to both poles ; apical disc large, genital plates of unequal size ; 

 anal opening large and excentral ; mouth opening large ; peristome notched into nearly 

 equal-sized lobes. 



Dimensions. — Height, nearly one inch ; transverse diameter, one inch and three tenths 

 of an inch. 



Description. — This is the only Hemicidaris known from the Portland Beds, it is identical 



* 'Historia Animalium Anglise Lap. Turb.,' p. 221, pi. 7, fig. 21, 15/8. 

 t ' Natural History of Oxfordshire.' 



