1886.] NATURAL SCIENCES OF PHILADELPHIA. 133 



Geological Position, etc Restricted to the Lower Silurian of 



America. 



We place in this genus the following species : 



*iS(i. Ohiocrinus constrictus (Hall), Heterocrinus constrictus, 24th Rep. N. Y. 



St. Cab. Nat. Hist., ]>. 210; also Geol. Surv. Ohio, Paleont., vol. i, p. 3, PI. 



i, figs. 10 a h; W. k Sp., Rev. I, p. 15. Hudson River gr. Cincinnati, 



(Ihio. 

 I8t5(). 0. laxus (Hall). Type of the genus. Heterocrinus laxus, 24th Rep. N. Y. 



St. Cab. Nat. Hist., p. 211, pi. 5, fig. 15; also Greol. Suiv. Ohio, Paleont., 



vol. i, p. 14. PI. i, figs. 8 h.\N. & Sp., Rev., I, p. 70. Hudson River gr, 



Cincinnati, Ohio. 

 *1882. 0. Gehanus (ririch), Heterocr. (locrinus) oehanus, Journ. Cincin. Hoc. Nat. 



Hist., vol. v, p. 175, PI. 5, fig. 9. S. A. Miller, 1883, Heterocr. oehanus, 



Cat. Amer. PalaMiz. Foss. (Edit, ii), p. 283. Hudson River gr. Cincinnati, 



Ohio. 



lOCRINUS Hall. 



1879. Revision of the Palseocr., Pt. I, p. 70. 



1882. P. Herb. Carpenter, Quart. Journ. Geol. Soc. London, p. 306. 



1883. W. & Sp., Amer. Joiim. Sl-i., vol.xxvi (Novbr.), pp. 370, 370. 



The genus locrinus must be slightly modified, as stated in our 

 recent notes " on Hybocrinus, Baerocrinus and Hoplocrinus.^'' 

 In that paper, to which we refer for further particulars, we gave 

 at some length the reasons which impelled us to regard the large 

 plate, which we had formerly called a radial, as representing the 

 azj^gous plate, and the smaller pentangular plate above, as the right 

 posterior radial. 



Walcott discovered lately a new generic form,- for which he pro- 

 posed the name Merocrinus. It agrees with locrinus in the 

 structure of the azygous side, and would probably be identical 

 with that genus if it had not well-developed underbasals. That 

 underbasals are entirely absent in locrinus is plainly shown by 

 its strictly pentagonal column, in which the angles have a radial 

 position. 



Revised Diagnosis. Dorsal cup broadly spreading and per- 

 fectly sj'mmetrical to the top of the second circlet of plates ; it 

 is short, and resembles an inverted pentagonal pyramid with the 

 five sides deeply concave. 



Basals small, nearly equal. Radials five, four of them of equal 

 form and height, comparatively large, strong and pentagonal ; 

 their upper sides truncated throughout their entire width for the 

 reception of the brachials. Adjoining these radials, in the same 



