1879.] NATURAL SCIENCES OP PHILADELPHIA. 283 



(1855, Lethsea Geognostica, Ausgabe III., Period I.), who went 

 to the opposite extreme by uniting Miller's typical species of 

 Cyathocrinus with Poteriocrinus, and placing the latter with 

 Woodocrinus, Homocrinus , Dendrocrinus, 31espilocrinus, and 

 Thysanocrinus under the Poteriocrinidae. It has been explained 

 elsewhere that Roemer adopted the generic name Cyathocrinus 

 with Miller's Cyathocrinus tuberculatus as type for another group 

 of Crinoids, which had been previously separated by Phillips 

 under Taxocrinus, and ranged these with Ichthyocrinus and a 

 number of other genera under the family name " Cyathocrinidse." 

 It will thus be understood that Roemer's Cyathocrinidae really 

 represent our Ichthyocrinidae, and his Poteriocrinidae our Cyatho- 

 crinidae. He united in the former an assemblage of very distinct 

 groups, and among his Poteriocrinidae are found Mespilocrinus, 

 which, as we believe, belongs to the Ichthyocrinidae, and Thysa- 

 nocrinus, which is distinct from any of these families. 



Angelin, in his systematic arrangement of the Gotland Crinoids 

 (Iconogr. Crin. Suec), very correctly places Sicyocrinus, Eus- 

 pirocrinus, Ophiocrinus, and Botryocrinus among the Cyathocri- 

 nidae ; but Gissocrinus, which closely agrees with Cyathocrinus, 

 except in having three underbasals instead of five, be classes (ap- 

 parently on account of this structure alone) with the Forbesio- 

 crinidae. We have already, in our introductory remarks, noted 

 the difficulty of classifying the genera according to the number 

 of proximal plates, and in Gissocrinus we have a good example 

 of this. 



A close and comparative stud}' of the genera Cyathocrinus and 

 Poteriocrinus has convinced us that, though the two are very dis- 

 tinct genericall}', they are, as between themselves and in connec- 

 tion with many other genera, united by very important structural 

 features, and by right ought to be regarded as of one family. 

 They all agree 



1. In having large oral plates supporting^the ambulacral grooves 

 and covering the ventral disc, but leaving an opening at the oral 

 centre, which is perfectly covered by the apical dome plates. 

 Food grooves along the vault closed by two rows of alternating 

 pieces. 



2. In the presence of a porous ventral sac, located posteriorly, 

 and closed at the top, in which the anal functions were subordi- 

 nate to other offices, probably in connection with respiration and 



