50 PEOF. P. M. DTJNCAJ!f's EETISION OE THE 



grooved. Spines large, some needle-shaped, others club-sliaped 

 and longer than the test's diameter, some knob-ended, others 

 forked, longitudinally striated, some marked across. 



Fossil. Zechstein, Alpine Trias, Lias, Oolites, Cretaceous : 

 Europe. Oolites : England. Jurassic, Tithonian, and Cretaceous : 

 N. Africa. 



This important genus has several fairly well marked subgenera 

 (formerly genera). It appears that the genus ILemipygus, Etal- 

 lon, 1850, Etudes Pal. Haut-Jura, suppl. p. 4, relates to small, 

 young, and immature Semipygi. As is usual in Echinoidea only 

 4 millim. in height and 8 millim. broad, the apical system is exag- 

 gerated in size and the genital perforations are variable. It is 

 placed as a synonym of Semicidaris. 



Subgenus Hemidiadema (genus), Agassiz, 1840, JEcJi. Suisse., 

 pt. ii, p. 47. Malloji, 1859, LetJicea JBrvMt. p. 326. 



The tubercles of the ambulacra large and few in number below 

 the ambitus, alternating distinctly. 



Fossil. Oolites and Cretaceous : Europe. 



jET. stramonium, Agass., is the type ; but H. rugosum does not 

 belong to the genus, it is a GlypJiocypJius, Haime. 



Many of the species of the next subgenus have been determined 

 from immature specimens ; this is especially the case with the 

 St.-Cassian forms. 



Subgenus Htpodiadema (genus), Desor, 1858 (pars), Synopsis, 

 p. 61. Lauhe, 1865, Denies. Akad. Wiss. Wien, vol. xv. 

 p. 295. Zittel, 1879, Sandh. d. Fal. p. 501. 



Ambulacra narrow, straight, with two rows of small, crenulate, 

 perforate primaries (?), nearly maintaining their size throughout, 

 diminishing but slightly abactinally. Interradia with large primary 

 tubercles, in two vertical rows extending to the apex; plates 

 granular beyond the scrobicules. 



Peristome and the branchial incisions small. Spines cylindrical 

 and smooth. 



Fossil. Oolite : England ? Trias, Oolites, and Cretaceous : 

 Europe. 



The Cretaceous species have variable branchial incisions, and in 

 one the primary interradial tubercles do not reach the apex. 



