422 CAftBONlFKIMUS FORMATIONS AND FAUNAS OF COLORADO. 



Myai.ina wYOMiNGENsrs Lea. 



ri. VIII, tiff's. 8, il, 10, 11, 12, 18. 



ISii'.i. Modiola wi/omiriflensh. Lea, AcaJ. Nat. Sci. Philadeliihiii, .Idur., (2), vol. 2, p. 205, pi. 20, fig. la. 



Coal Formation: Wilkesbarre, Pa. 

 18S(i. Modiola wj/omiiuiensis. Claypole, Proc. and t!oll. AVyoniing Hist, and (ieol. Hoc, vol. 2, pt. 2, 

 p. 247. 



Lower Coal Mca.sures: Wilkesbarre, Pa. 

 1898. Mi/alhia wyominoensi.i. Weller, I^. S. Geol. Surv., Bull. No. 153, p. 365. 



Shell (if niediuin .size; .shape, subtriangular to subquadrangular. Hinge line 

 .straight, long; beak nearlj' but not quite terminal. Umbonal ridge well marked, 

 meeting the hinge line at an angle of about 45°. Anterior outline .sinuate, converg- 

 ing below with the umbonal ridge, which becomes less elevated in this direction. 

 The posterior outline is often nearly straight in its upper portion and parallel with 

 the anterior margin, but it is more .stronglj^ curved below and its junction with the 

 anterior outline is subangular. The shape of such a specimen resembles that of a 

 parallelogram. In some examples, however, the hinge line is projDortionately longer, 

 and the posterior margin makes a more or less regular, obliquely directed curve 

 from the hinge line to its union with the anterior outline. The shape resulting is 

 subtriangular, and all gradations exist between the two. Variation precisely similar 

 is seen in Myalina pei'miana Swallow. The elongated form Swallow describes as 

 M. pennlwna; to the triangular one he gave the name M. concava. This fact has 

 been pointed out bj^ Meek and Hayden," who place the latter .species in the syn- 

 onymy of the former. I do not consider this variation of specific importance in the 

 case of M. toyomingeoisis, and certainly it seems no more than varietal in the case of 

 M. permiana. 



The surface seems to be niarked by lamellose striae, which are particularly^ prom- 

 inent on the peripheral areas of the shell. 



This species is closely related to M. pet^miana and shows many points of resem- 

 blance. At first, indeed, I identified my material with Swallow's species with but 

 little reserve, but a critical revision has compelled me to recall this decision. The 

 difi'erences which distinguish them reside chiefly in the anterior portion of the shell. 

 In M. jiermiana the umbones are terminal and the umbonal ridge is so near the 

 anterior outline that where the chief emargination of the latter occurs the umbonal 

 ridge actually overhangs the shell margin. In M. uiyomingeiisis the umbones are 

 not quite terminal and there is alwaj's a more or less well-developed anterior lobe. 

 The anterior outline is farther from the umbonal ridge in the upper part of the 

 shell, and the anterior side there falls away less abruptly than is the case below, 

 where, though the ridge is not so high, it slopes ofl' to the plane of the shell edge in 



"Pmith.^ionian Cont. Knowledge, vol. 14. Xo. M-2, 1885. p. h'2. 



