286 CATALOGUE OF THE BLASTOIDEA. 



Billings regarded Codaster as a Cystid and not as a Blastoid because of the 

 absence of ambulacral pores, and he naturally, therefore, took a similar view with 

 respect to Orophocrinus. Meek and Worthen, however, seem to have regarded it as 

 belonging to the Blastoidea ; and it was definitely placed in this Class by Zittel, with 

 whom we entirely agree as to this point, but we do not think that it is so closely 

 allied to Pentremites as was supposed by Meek and Worthen. An approach to the 

 form of the true Pentremite is, however, exhibited by one species in particular, the 

 0. gracilis, M. & W. 1 , sp., more than by any of the others. Although possessing the 

 characteristic features of Orophocrinus, the upper portion of the calyx is much 

 higher and less depressed than in the typical 0. stelliformis. 



The genus Mitra, Cumberland 2 , is undoubtedly in part congeneric with Oropho- 

 crinus, and had the name not been preoccupied by Lamarck as a genus of Mollusca, 

 it would have been necessary for us to adopt it. D'Orbigny's Dimoiphicrinus is 

 likewise identical with von Seebach's type. In the original definition 3 it is said to 

 have only two rows of plates, but de Koninck and Le Hon have since stated that 

 Platycrinus pentangularis, Miller, on which it was founded, is a true Pentremites; 

 and we have no doubt that one of the two specimens figured by Miller 4 under this 

 name is an imperfect example of an Orophocrinus. 



The deltoid plates, except in well-weathered specimens, are difficult to see, but are 

 usually acutely arrowhead-shaped in 0. ptentangularis (PI. XV. figs. 6,10), 0. gracilis, 

 and 0. Orbigntjanus, or double diamond-shaped, and constricted in the middle, as in 

 0. stelliformis (PL XL figs. 8, 9). The anal aperture is large, and either oval, as in 

 the last-named species (PL XV. fig. 11), or elongately and roundly triangular. In 

 0. stelliformis and 0. vents (PL XVI. fig. 10) it also has an outer raised margin, but 

 we have not observed any tube or small proboscis as described by Messrs. Wachsmuth 

 and Springer 5 . 



The ambulacra are narrow in all but 0. stelliformis and 0. Orbignyanus, where 

 they broaden out (PL XIV. figs. 16-18 ; PL XV. fig. 11). We have not succeeded 

 in detecting any sign of outer side plates, unless it be in 0. Orbignyanus, but of this 

 we cannot be sure (PI. XL fig. 10). Meek and Worthen say they are unknown in 

 0. stelliformis (PL XV. fig. 14), but they appear to figure them in 0. gracilis 

 (PL XVIII. fig. 9). The side plates are relatively very large, however, and their 

 apposed edges are notched so as to form sockets for the reception of the ambulacral 

 appendages, just as in Codaster (PL XIII. figs. 14, 16 ; PL XV. figs. 7, 14). 



The upper parts of the lancet-plates are exposed in all the species, but towards their 

 distal extremities the side plates often meet in the middle line and close over them, 

 the amount of covered surface varying according to species. The side plates are 



1 Report Geol. Survey Illinois, 1873, vol. v. t. 8. f. 6. 2 lleliquia; Conservatse, 1826, p. 31. 



3 Trod. Pal. Strut. 1849, i. p. 155. 4 Op. at. p. 81, fig. 2. 



■' ' Revision of the Palceocrinoidea,' Part II. 1881, p. 18, note. 



