1879.] NATURAL SCIENCES OP PHILADELPHIA. 315 



s 9. ARACHNOCRINUS Meek & Worthen. 

 ' 1866. Geol. Rep. Illinois, -vol. ii. p. 177. 



The above name, with Cyathocrinus? bulbosus Hall as type, 

 was proposed by Meek and Worthen for a little group of Crinoids 

 which are distinguished by their spider like appearance. The 

 calyx in these species, as compared with the long and robust arms, 

 is extremely small, hemispherical, and forms a bulb-like protuber- 

 ance, from which the arms spread horizontally. These characters 

 distinguish this form readily from Cyathocrinus, with which it 

 otherwise agrees, and they are so remarkable that we feel con- 

 strained to consider Arachnocrinus a distinct genus, and not a 

 subgenus, as suggested by its authors. 



We had lately the pleasure of examining the splendid collections 

 of Dr. Knapp in Louisville, where we found two exceedingly in- 

 teresting specimens, representing different species of this genus, 

 both new. In one, the arms are almost closed, in the other spread 

 out horizontally, thus demonstrating clearly that this Crinoidwas 

 capable of folding up its arms, and bringing them to an erect posi- 

 tion, which has been doubted by Meek and Worthen, who could 

 not conceive how an animal with so small a visceral cavity could 

 have sufficient muscular power to move such ponderous arms. Dr. 

 Knapp kindly permits us to describe the two species, which will 

 be known as A. Knappi and A.extensus. Roemer's Poteriocrinus 

 pisiformis, which Meek and Worthen refer to this genus, is but 

 imperfect^ known, but we do not doubt that the reference is 

 correct. We also place in this genus Cyathocrinus granulatus 

 Angelin, though it differs from the other species in the brachials, 

 which are restricted to one to each ray. We propose the follow- 

 ing: 



Generic Diagnosis. Calyx very small, resembling Cyathocrinus 

 in the arrangement of the plates, but more depressed ; basal por- 

 tion rounded. Arms exceedingly robust, generally stretched out 

 horizontally, and arranged around the body, as the arms in ^4,s- 

 trophyton around the disc. 



Underbasals five, small, apparentl}' of equal size. Basals five, 

 the posterior one truncate. Radials wider than high, articulating 

 scar occupying almost the entire width of the plate, and more or 

 less concave. The number of brachials differs in the rays, as many 

 as fourteen having been observed - T they are cylindrical, wider than 



