GENEBA AND GROUPS OF THE ECHINOIDEA. 93 



Genus Aebacia, Gray, 1835, Froc. Zool. Soc. p. 58. Troschel, 

 1872, Arch. f. Naturg. vol. sxxviii. p. 293 ; 1872, vol. 

 xxxix. p. 308. A. Agassiz, 1874, Bevision, pp. 263, 399. 

 Bell, 1879, ProG. Zool. Soc. p. 436. Duncan Sf Bladen, 

 1885, Journ. Linn. Soc. p. 48. Loven, 1887, JEch. descr. ly 

 LinncBUs, p. 80. (^Amended.) 

 Syn. Echinocidaris, Desmoulins, 1835, Liitken, 1863 ; Agarites, 



Agassiz, 1841 ; Pygomma, Troschel, 1873. 



Test moderate, thick, subcircular at the tumid ambitus, sub- 

 hemispherical, depressed dorsally, flat actinally. 



Apical system large, prominent ; basals projecting into the 

 interradia, long ; radials- entering or not the ring, the pore 

 adoral double ; all plates ornamented with epistroma ; periproct 

 oval, oblique, with four triangular periproctal plates (sometimes 

 three to five). 



Ambulacra straight, narrow to the ambitus and larger there 

 and at the peristome and with a projecting lip ; poriferous 

 zones moderately narrow, and the pairs of pores in simple pairs 

 dorsally, then in arcs of three and polyserial at the peristome ; 

 plates low primaries dorsally, then compound with a large median 

 plate and an adoral and an aboral demi-plate, their inner sutures 

 nearly vertical and curving upon the tubercle. Tentacles hetero- 

 podous. 



Interradia with bare median dorsal areas near the apex, elsewhere 

 with from three to seven vertical rows of plain primary tubercles 

 on each side of the median line, more or less oblique, larger than 

 those of the two vertical rows of the ambulacra, all diminishing 

 in number and size dorsally. Epistroma greatly developed. 

 Secondary tubercles absent. 



Peristome large, tags or lamellae for the branchise ; ten buccal 

 plates. Perignathic girdle with ridges, the ambulaeral processes 

 moderate but not arched. Jaws with the foramina of the 

 pyramids open ; teeth with a keel. Spines long, moderately stout, 

 some spathiform, cellular within, may have a " cap." Pedicellarise, 

 some with large glands on their stalks. Plates of the test 

 united at the vertical sutures by partial dowelling, some of it 

 coarse. A solitary spheridium in each ambulacrum in a large pit. 



Fossil. N. America, Tertiary ? (The European Miocene spe- 

 cies is a PsammecMnus.) 



Pecent. Florida, Caribbean Sea, Long Island to Yucatan, 

 Brazil, Straits of Magellan, Patagonia, Chili, Peru, Panama, 



