Maryland Gi:ulogxcal Survey 589 



This species differs frmn A. rfficuliirls in its c(>;U'ser, more iindose pli- 

 cations. 



Occurrence. — Jexxixgs FouiiATiux, Ciiemuxg ]tlE.MBEC. Oaldand- 

 Eedhouse Eoad; Trout Eun section : Williams Eoad, on Polish ^ilountain, 

 24S:!, 2708; National Eiia<l. on Polisli :\[ountain, 270(i, 2949; Town 

 Creek, 3384; Ellerslie, Pennsylvania, 1316, 1474 abundant, 1519 abun- 

 dant. WooDMOXT Member, Ithaca Fauxa. Two miles west of Pawpaw. 

 West Virginia, 1388. 



CoUectioit. — ]\rar\land Geological Survey. 



Atrypa hystrix Hall 

 Plate LV, Figs. 14-19 



Atrypa hystrix Hall, 1843, Geol. of N. Y., Rept. 4th Dlst., p. 27, fig. 2. 

 Atrypa hystrix Hall, 1867, Pal. of N. Y., vol. iv, p. 32G, pi. lilia, figs. 15-17. 

 Atrypa hystrix Grabau and Slilmer, 1909, N. Anier. Index Foes., vol. 1, p. 311, 



fig. 392. 

 Atrypa hystrix Cleland, 1911, Bull, xxi, Geol. Survey, Wis., p. 73, pi. xiil, 



figs. 14-16. 



Description. — Associated with the usual form of Air/j/jn reticularis 

 already considered occur Atrypas of large size with broad and few plica- 

 tions crossed by projecting concentric lamella. These lamellte present the 

 aspect of a crenulated frill and where they cross the plications project out- 

 ward as a free fold, or these expressions may be so infolded at the sides as 

 to produce hollow spines. The spinous shell is the typical expression of the 

 original A. hystrix of the Chemung and the A. spinosa of the Hamilton 

 stage,' while to the spineless but scraggy shell of the Xew York Chemung 

 with these few plications Hall originally applied the European (Sehlo- 

 theim's) term aspera. subsequently calling the Iowa shell, which is rela- 

 tively plumper and more gibbous, A. aspera var. occidentalis. With a 

 large number of representatives of all these expressions of surface it is diffi- 

 cult to determine how the names are to be restricted, as the passage of the 

 frilled lamellae into hollow spines can often be obseiwed in the individual 

 slull and yet in the main tlie expressions of the shells indicate their 



'Tlie allied form from the middle Devonian of Iowa, A. hystrix var. occi- 

 dentalis Hall, is described and figured in Geology of Iowa. 1858, vol. i, pt. ii, 

 p. 515. pi. vl, fig. 3. 



