122 Dr. T. Wright on a new Genus of Fossil Cidaridse, 



area, stop short at the. equator, or between the equator and the 

 disc ; the intertubercular space on the upper surface of the test is 

 therefore in general wide, and covered with a small miliary gra- 

 nulation. The apical disc is large ; the genital and ocular plates 

 are expanded and foliated. The mouth-opening is of moderate 

 dimensions, and the peristome is divided into ten nearly equal- 

 sized lobes. The spines are long, slender, and needle-shaped ; 

 those that are known, equal at least the diameter of the test, and 

 their surface is sculptured with delicate longitudinal lines. 



Hemipedina is related to Diadema in having the pores ar- 

 ranged in the zones in single pairs and the tubercles perforated ; 

 but it is distinguished from Diadema by the absence of crenu- 

 lations on the summits of the mammillary eminences. It is 

 related to Pedina in possessing perforated and uncrenulated 

 tubercles ; but it is distinguished from that genus in having the 

 pores in the zones in single pairs (Pedina having the pores in 

 triple oblique pairs like Echinus), in having the elements of the 

 apical disc more largely developed, and in the species being nearly 

 all small and depressed forms. Hemipedina is related to Echin- 

 opsis in possessing uncrenulated and perforated tubercles, with 

 the pores in pairs ; but it is distinguished from the latter by the 

 narrowness of the ambulacral areas, the depressed form of the 

 test (Echinopsis being high and inflated), the form of the mouth- 

 opening, and the deep decagonal lobes of the peristome (that of 

 Echinopsis being almost deprived of these incisions), together 

 with the greater size and development of the elements of the 

 apical disc. 



Hemipedina, as far as we at present know, is composed of 

 Jurassic species, which commence in the lower Lias and extend 

 into the Coral Rag, each stage possessing its own specific forms. 

 The following synopsis of the British species now before us will 

 be figured and described in detail in our Monograph on the 

 British Oolitic Cidaridse. 



A. Species from the Lias. 



Hemipedina Bechei, Wright. 



Cidaris Bechei, Broderip, Geol. Proc. ii. 202. 



Diadema Bechei, Agassiz, Morris's Catalogue of British Fossils, 

 1st ed. p. 51. 



Test small, much crushed, and covered over with spines ; ambu- 

 lacra with two rows of tubercles; interambulacra with four 

 or six rows of tubercles ; spines long, slender and needle- 

 shaped, T 8 ths of an inch in length, with longitudinal lines on 



