DESCRIPTIONS or Tin; Sl'Kl IKS. 291 



justice to one of the earliest workers in British Palaeontology we adopt his name 

 for the species in question, which forms the British typo of the genus OropKocriwus. 



Prof. F. Roeiner 1 was the first to suggest the necessity of separating P. inflata, 

 Sby., from the typical Pentremites on account of the absence of outer side plates in 

 the ambulacra ; and the want of hj drospirc-pores was subsequently pointed out by 

 Rofe 2 , who took the linear spiracular clefts to be mere grooves in which the 

 pinnules were inserted, and stated in consequence that this species, together with 

 " P. Waterhousei (de Koninck) differ from all the above (?'. e. American Pentremites) 

 as they do not show the summit-openings from which this genus derives its name ; 

 but they have one large circular opening at the summit of one of the quadrilateral 

 plates, in the place usually occupied by the largest of these openings, and which is 

 shown by Prof. Forbes to be anal." 



The resemblance of the external form of this species to that of Orophocrinus 

 stellifonnis was noticed by Meek & Wortken 3 when they made the latter the type 

 of their new genus Codonites, but in the absence of good figures of 0. verus, they 

 were unable to carry their comparison any further. The resemblance in general 

 character between 0. verus and 0. stelliformis is a very marked one ; but the 

 proportions of the component parts of the calyx are different, so that the former has 

 a less lobate outline, narrower sinuses, a more convex summit, and longer spiracles 

 (PI. XV. figs. 1-4, 11, 13). The side plates in 0. verus are numerous, but less so 

 than in 0. stelliformis, which is said to possess fifty on each side of an ambulacrum, 

 whilst the former has only from forty to forty-five. On the other hand, both species 

 are alike in their anal apertures having thickened and prominent distal margins 

 (PI. XV. figs. 3, 11 ; PI. XVI. fig. 10). In 0. pentanrjularis, Miller, sp., the calyx 

 is much longer, the ambulacra shorter and less convex, and the summit more 

 constricted (PI. XV. figs. 5, 6, 8, 9). We give a very instructive figure of an 

 ambulacral field of 0. verus, with the side plates and lancet-plate entirely removed, 

 so as to show the under lancet-plate (PI. XV. fig. 4). The hydrospire-clefts, which 

 are visible externally along the sides of the ambulacra, only communicate internally 

 with the hydrospires by their pyriform proximal ends. In the allied species, 

 0. pentangularis, the extension and enlargement of these openings as compared with 

 those of 0. verus is very noticeable (PL XV. fig. 10), while they are smaller in the 

 American 0. stelliformis (PI. XV. fig. 13). 



We figure two well-marked varieties of 0. verus, in which the difference in the 

 relative heights of the calyces, between the base and the radial lips, is well shown. 

 In the first (PI. XV. fig. 3) the ambulacra are relatively long whilst those parts of the 

 calyx which lie below the lips are proportionately reduced. In the other specimen 



1 Archiv f. Jsaturgesch. 1851, Jahrg. xvii. Ed. i. p. 357. 



2 Geol. Mag. 18G5, vol. ii. p. 250. 



3 Report Geol. SurTey Illinois, 1873, vol. v. p. 4UG. 



2p2 



