1873.] NATURAL SCIENCES OP PHILADELPHIA. 369 



three ; parabasals five, polygonal. Primary raclials one, forming 

 a single zone; secondary raclials in two series, the upper ones 

 triangular and arm bearing. Interradials (probabty anals) two. 

 Arms long, threadlike, dichotomizing several times, composed of 

 single joints. Ventral tube distinct, articulated. Column strong, 

 convoluted, composed of thin joints with numerous articulated 

 cirrhi." He places this genus under his division "Trimera" 

 (along with Taxocrinus and Gissocrinus), among the Taxocrinidae. 



Comparing Angelin's figures, we find that only his M. hete.ro- 

 crinus agrees with the description. His M. gracilis lias five 

 primary raclials, or, as we should sa_y, four brachials ; his M. (?) 

 inter radial is three to four, and, contrary .to the rule in all Cyatho- 

 crinidse, a number of interradial plates. None of the species, 

 however, exhibit the peculiar arm structure upon which Hall 

 founded the genus; but, on the contrary, the two first-named 

 species at least seem to be devoid of pinnulse. Angelin figures pi. 

 10, fig. 25 an isolated convoluted column with numerous cirrhi, 

 which is said to belong to M. gracilis, and it seems that the super- 

 ficial resemblance of this column to the so-called arms of Myelo- 

 dactylus induced Angelin to adopt Hall's name for his species. 

 We cannot see any propriety in founding genera or species upon 

 mere fragments of arms or column, especially in a case like this, 

 where we are by no means satisfied that Hall's figure represents 

 the column, but rather believe, with him, that they are most prob- 

 ably portions of arms. We have here placed Myelodactylus pro- 

 visionally under the Cyathocrinidae, because Anuelin's first two 

 species undoubtedly belong to that family, though prohably to 

 different genera. His M. (?) inter radialis, however, is a repre- 

 sentative of an altogether different family. They all differ from 

 the Ichthyocrinidse in the large underbasals, in the free plates 

 above the first raclials, the threadlike arms, and the general physi- 

 ognomy. So far as we know, the species agree with no established 

 genus, but the figures without descriptions are not sufficient for 

 us to found new genera upon. 



Hall places here the following species : 



1851. Myelodactylus brachiatus Hall. Geol. Surv. N. Y., Pal., vol. ii. p. 232, pi. 



45, fig. 7. Niagara Gr. Upper Sil. Near Lockport, N. Y. 

 1851. Myelodactylus convolutus . Geol. Surv., N. Y., Pal., vol. ii. p. 19], 



pi. 42, figs. 5 a, b, and 6 a-h. Niagara Gr. Upper Sil. Lockport, N. Y. 



