DESCRIPTIONS OF THE SPECIES. 236 



is due to the concealing of the hydrospire-canal by the lancet-plate, which cornea into 

 direct contact with the deltoids, except at the spiracles; whereas in Acentrotremites 

 the hydrospire-canal opens externally through the spiracles at the aboral ends of the 

 deltoids, and is not therefore continued onwards along their ambulacra! edges as it is 

 in Oryptoblastus. At any rate, this is what we should infer from the condition of 

 the specimen represented in PL XIII. figs. 18, 19, which seems to indicate that the 

 proximal ends of the hydrospire -tubes are situated at a point much lower in the 

 calyx than is generally the case in the Blastoidea. It is quite possible, however, that 

 the hydrospire-slits may extend upwards on to the deltoids within the ring of spiracles 

 without having any communication with the exterior through pores at the sides of 

 the ambulacra. There are various other Blastoids, however, in which the hydrospire- 

 slits are practically limited to the radials, scarcely any portions of them extending 

 upwards on to the deltoids, e. g. Granatoerinus Xonvoodi, so far as can be judged 

 from the condition of the infernal casts (PI. VII. figs. 7-9), and Orophocrinus (PI. XV. 

 figs. 4, 10). 



The last-mentioned genus resembles Acentrotremites in having a distinct anal 

 opening and ten groups of hydrospires. The enlarged proximal ends of the clefts 

 by which these open externally, represent the spiracles of Acentrotremites ; and in 

 the best-known species (0. steUiformis, PI. XL fig. 9) they occupy very much the 

 same position as their homologues do in that genus, viz. at the junction of the 

 radio-deltoid sutures with the ambulacra. But the other characters of Orophocrinus, 

 and especially those of the European species, are such as to separate it entirely from 

 Acentrotremites. The same may be said of Metablastus and Tricoelocrinus (PL III. 

 figs. 14, 15 ; PL XIX. figs. 15, 16), which also have an anal opening and ten spiracles, 

 while Elceacrinus is distinguished by its anal plate. 



Species. When we first described Acentrotremites from the characters of Cumber- 

 land's Mitra elliptica, we also referred to this genus a fragmentary specimen from 

 Clitheroe in the Rofe collection (PI. IV. fig. 3). But we have since seen some better- 

 preserved examples of the latter type, and find that it is really a large species of 

 Mesoblastus, which we shall describe hereafter as M. Itofei. 



Distribution. The only known species of Acentrotremites is confined to the Carboni- 

 ferous Limestone of Britain. 



Type. Mitra elliptica, Cumberland. 



Acentrotremites ellipticus, Cumberland, sp. 

 (PI. XIII. figs. 17-19.) 



Mitra elliptica, Cunib., Reliquiae Conscrvatre, 1826, p. 33, t. B. f. 1-3 (middle group) [non 

 Pentremites elliptica, G. B. Sby.]. 



Sp. Char. Calyx large, tapering upwards ; base flat, or very slightly concave ; sides 



2 h 2 



