286 PROCEEDINGS OF THE ACADEMY OP [1819. 



Zeacrinus type : 



16. Woodocrinus de Koninck. 18. Hydreionocrinus de Koninck. 



17. Zeucrinus Troost. Subgenus(?) Cceliocrinus White. 



Transition forms toward Encrinus : 



19. Eupachycrinus Meek & Worth. 21. Stemmatocrinus Trautschold. 



20. Erisocrinus Meek & Worth. 



Genera insufficiently known : 



22. Euxpirocrinus Angelin. 25. Pacfiyocrinus Billings. 



23. Carabocrinus Billings. 26. Myelodactylus Hall. 



24. Cyriidocrinus Angelin. 



In some species of Heterocrinus, one of the oldest of known 

 genera, the underbasals or proximal plates are apparently want- 

 ing; in others they are so imperfectly developed that Meek did 

 not think proper to recognize them by the usual term, but called 

 them sub-basals. If it happens that these plates are absent, as 

 seems to be sometimes the case, in species which apparently belong 

 to the same genus, it would seem improper to separate Hetero- 

 crinus on that ground alone, and this consideration has induced 

 us also to place in this family, at least for the present, the allied 

 genera Hybocrinus and Anomalocrinus, in which as yet no trace 

 of these plates has been discovered, but which otherwise have all 

 the characters of the typical Cyathocrinida?. The family re- 

 lations as to these genera are not altogether clear, and it will re- 

 quire further study and better material before we can expect to 

 understand them fully. For the present we will content ourselves 

 with drawing attention to some peculiarities in the structure of 

 these early Crinoids in hope that it may lead eventually to in- 

 teresting results. 



We find that in Poteriocrinus, Zeacrinus, and in fact in all typi- 

 cal Cyathocrinidae in which columnar or radicular cyrrhi have 

 been observed, these appendages are radially situated. This is 

 best observed and traced in species with a pentagonal stem, 

 wherein the direction can be noted even in fragmentary pieces. 

 The cirrhi here occur always along the lateral faces, and in the 

 last-named genera, so far as observed, they are alwaj^s radial, 

 while the acute angles which are formed into elevated ridges along 

 the column are interradial. The A r ery opposite is the case in 

 Heterocrinus and Iocrinus, in which the cirrhi are interradial, and 

 the ridges of the column radial, exactly as in Belemnocrinus, and 



