1879.] NATURAL SCIENCES OF PHILADELPHIA. 291 



4. HYBOCKINUS Billings. 

 (Diagram PI. 16, Fig. 4.) 



1856. Geol. Surv. Canada, p. 274, and Dec. IV. p. 23. 



('Apiocrinites Leuchtenberg, Haplocrinus Grewingh, Homocrinus, p.p. 

 Eicliw., Bcerocrinus Volborth, are synonymous. See Zittel's Handb. d. 

 Petrefactenkunde, p. 350.) 



Catyx globular or pj-riforni, one side protuberant ; composed 

 of 5 basals, 5 radials, and 2 anal plates. 



Underbasals not observed, and probably rudimentary. Basals 

 of equal size, pentagonal. The next ring of plates consists of a 

 large anal and four of the radials, all nearly equal in size, and 

 alternating regularly with the basals. The anal plate is hexa- 

 gonal, its two upper sides equal, the sloping right side supporting 

 a small radial, the left a second anal plate. Both of these plates 

 are wider than high, of about the same size,' which is about one- 

 third that of the plates below. They are separated by a vertical 

 suture, and rest by their outer edges against the upper portion of 

 the adjoining radials which are octagonal, while the other two are 

 heptagonal. 



Arms five, simple throughout, composed of rather heavy quad- 

 rangular joints, about as wide as high, decreasing in size slightly 

 upward. Pinnulse wanting. The ambulacral furrow is covered 

 by small alternating pieces, about five to each arm-joint. 



Column round, small. 



The unsymmetrical form of the calyx, produced by the protu- 

 berance of the posterior side ; the peculiar position and small size 

 of the right posterior radial, and the large anal plate in line with 

 the four larger radials, are the most remarkable features of this 

 genus, and those by which it is easily recognized. The small 

 radial evidently corresponds to the upper half of the compound 

 plate in Dendj'ocrinus, while the lower half, which is here appar- 

 ently absent, is perhaps represented in a portion of the large un- 

 divided anal plate. 



Geological position. Lower Silurian, and so far found only in 

 America. 



The following species have been discovered: 



1856. Hybocrinus conicus Billings (type of the genus), Geol. Surv. Canada, p. 274 ; 

 also Decade IV. p. 29, pi. 2, fig. 2 a, b. Trenton limestone. Ottawa, 

 Canada. 

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