GENEKA AND GROUPS OF THE ECIlIlsrOIDEA. 153 



Fossil. Tertiary : Europe, Africa, Asia, Australia, and N. 

 America. 



Becent. fled Sea, East-Indian Islandti, New C aledonia, Philip- 

 pines, Japan, Kingsmill Is., China Sea, Panama, San Diego, West 

 coast of Africa, S. Carolina, Florida, Caribbean Sea, Brazil. 



The huge, tall, bulky Tertiary species have flat actinal surfaces 

 and are true Clypeasters. C. alius and C ceqyptlacus have short 

 stout ridges upon the ambulacral plates of the petals, within the 

 test, and they do not coalesce, or do so very imperfectly in places. 



Subgenus Monosttchia, (genus) Laube, 1869, Sitz. d. hais. 

 Ahad. d. Wiss. JVien, p. 188 ; (genus) Duncan, 1887, Quart. 

 Journ. Geol. Soc. vol. xliii. p. 417. 

 Test small, flat. 



Apical system small ; madreporite button-shaped ; four genital 

 pores ; margin notched when young decidedly, and the peristomial 

 ambulacral grooves straight and continued abactinally. 

 Fossil. Tertiary : Australia. 



G-enus Diplothecanthus. 



Syn. EchinantJius, Breynius, 1732, pars; Leske, 1778, pars; 

 Linnaeus, l76i, Mus. Lud. Ulrich. p. 714 {Echinus reticulatics, 

 see Loven, 1887, Ech. descr. by Linnaeus, p. 174) ; J. Miiller, 

 1853, Bau d. Echin., Abhandl. d. konig. Akad. d. Wiss. zu 

 Berlin, pub. 1854, p. 123 ; Gray, 1855, pars ; A. Agassiz, 1872-74, 

 Eevision, p. 310. 



Test moderate and large, pentagonal or subcircular in tumid 

 marginal outline, angles rounded ; dorsum tumid variably ; actinal 

 surface tumid, but hollow from the margin to a deep central 

 peristome. 



Apical system small, madreporite star-shaped, central; the 

 small radial plates at the re-entering angles, pores small, usually 

 transverse ; the basal plates fused ; genital pores in the median 

 sutures of the interradia, more or less remote. 



Ambulacra with long, broad, tumid, almost closed petaloid 

 parts, with the poriferous zones broad, slanting or depressed ; the 

 pores distant, unequal ; the plates compound, the adoral compo- 

 nent a large low primary, the aboral a small demi-plate ; the wide 

 interporiferous areas perforated ; beyond the petals the plates are 

 simple and larger, and are mainly perforated obliquely actinally. 



The interradia small abactinally and sunken ; discontinuous 

 actinally, one plate entering the peristomial margin in each. 



