the Morphology of the Bias to idea. 251 



spire-clerts in the latter much wider than in the former, but 

 they are also contiguous to the ambulacra, without tha inter- 

 vention of a part of the radial plate. 



D'Orbignj's Bimorphicrinus^'^ is undoubtedly congeneric 

 with Orophocrinus. In the original definition it is said to 

 have only two row^s of plates ; but de Koninck has since 

 shown t that it was founded on Platycrinites pentangularis^ 

 Miller, which he considered a Pentremite deprived of its 

 oral plates, whilst we hope to definitely show its relation to 

 0<i>i)!iocrinus. 



We agree with the late Mr. Billings in regarding the struc- 

 ture of Orophocrinus as of more than generic importance when 

 compared with that of other Blastoids ; and we follow Meek 

 and Worthen, as does Zittel, in regarding Orophocrinus as a 

 Blastoid and not a Cystidean, as it was asserted to be by 

 Billings :|:. He seems to have been led to this conclusion by 

 the discovery of specimens with the peristome closed by 

 minute plates, and by his peculiar views as to the mouth being 

 confluent with the anal aperture. The absence of hydrospire- 

 pores and spiracles we believe to be points of the greatest 

 structural importance within the Blastoidea, but not sufficient 

 in themselves to eliminate Orophocrinus from that class. 



The oral plates, except in well-weathered specimens, are 

 difiicult to see, but are usually acutely arrowhead-shaped 

 (0. Waterhousianus, O. gracilis, and 0. Orbignyanus)^ or 

 double diamond-shaped, with a constricted middle, as in 0. 

 sieUiforinis. The anal aperture is large, and either oval, as in 

 the last-named species, or elongately and roundly triangular. 

 In 0. stelliformis it also has an outer raised margin ; but we 

 have not observed any tube or small proboscis as described in 

 this form by Messrs. Wachsmuth and Springer §. 



The ambulacra are linear in all but 0. Orhignyanus^ where 

 they broaden out, and they possess only one groove, the food- 

 groove, not three as in CoJaster. We have not succeeded in 

 detecting any sign of outer side plates, unless it be in 0. Or- 

 hignyanus ; but of this we cannot be sure. Meek and Worthen 

 say they are unknown In O. stellifonnis '^ but they appear to 

 figure them in O. gracilis\\. The length of the hydrospire- 

 clefts is a variable character. They are shortest in 0. stelli- 

 formis^ but extend nearly the whole length of the ambulacra 

 in 0. Waterhousianus and 0. pentangular is , and c^nite that in 

 0. inflatus and 0. Orhignyanus. As before stated, no true 

 spiracles are present ; but the clefts enlarge upwards towards 



* Prodrome de Pal. 18-lU, i. p. lo5. 



t Rech. Criuoides Terr. (larb. Belgique, p. 19J. 



I Americau Jomn. Sc. 1870, i. p. 2o4. § Revision, pt. 2, 



II Iliiuois Geul. Siirv. Report, lb73, v. t. 8. f. 6, 



