170 PBOP. V. M. DTTNCAN's RETISIOIS" or THE 



Grenus Ambltptgus, Agassiz, 1840, Catal. 8yst. 'Ectyp. 'EeMn. 

 Mus.Neoc. p. 7. Z)esor, 1858, AS'y?iops^>, p. 255. Duncan^ 

 Sladen, 1883, Pal. Ind. ser. xiv., Foss. EcJi. KacJih ^ Kat. 

 p. 12. Duncan, 1884-, Geol. Mag. p. 17. 



Test large, stout, ovoid or subpentagonal at the tumid ambitus, 

 moderate or depressed in beiglit, tumid and convex dorsally, 

 more or less swollen actinally, around the peristome. 



Apical system central or subcentral, small, four basal plates 

 with large ducts, and perforated by tbe madreporite, which is 

 largely in the right antero-lateral basal, and which may pass 

 between the postero-lateral basals and even separate the pos- 

 terior radial plates. Eadial plates small. 



Ambulacra narrow, flush or slightly raised, apetaloid, conti- 

 nuous from the apex to within the peristomial tube ; poriferous 

 zones expanding below tlie apical system and contracting before 

 reaching the ambitus, thence becoming very narrow and con- 

 sisting of a straight series of oblique small pairs ; at the expanded 

 part the pairs are separated by a raised costa, the inner pores 

 are circular, the outer series being transversely elongate and the 

 pores are connected by a groove. Plates are mainly low primaries, 

 each with a pair of pores ; but many are composed of a small 

 demi-plate and a large primary combined. 



Interradia broad ; unequally broad at the peristome, where the 

 plates are single in all the areas except 2 and 4, in which there 

 are two peristomial plates ; all these plates pass up slightly and 

 form a kind of mouth-tube. 



Tubercles of both areas very small, crenulated, distant, and 

 separated by sharp epistromal granules. 



Peristome large, oblique, the long axis between interradia 2 

 and 4, sunken, deep, widely open, irregularly triangular, with a 

 long and short axis, subcentral. Periproct large, pear-shaped, its 

 long axis in that of the test, situated near the peristome and 

 between it and the posterior margin. 



Fossil. Cretaceous : Cuba (?). Eocene : Europe, E, Africa, 

 Asia. Tertiary : Jamaica. 



Grenus Caratomus, Agassiz, 1840, Catal. Syst. Ectyp. EcJi. Mus. 



Neoc. p. 7. Desor, 1842, in Agass. Monogr. d'Ecli. viv. etfoss. 



livr. 3, p. 35. Cotteau &f Triger, 1859, EcTi. de la Sarthe, 



pi. xxxi, figs. 15-19. 



Test small, depressed, ovoid or elliptical in tumid outline,- 



