DESCRIPTIONS OF SPECIES. 423 



a direction nearlj^ normal to the latter. The anterior lobe seems to be always more 

 or less developed, and though the extreme form in one direction comes close to 

 M. permiana, it is even then strong enough to differentiate the two. 



M. Wyoming ensis also resembles M. aviculoides, but a comparison of my material 

 with the type specimens of the latter leaves me convinced that the}^ are distinct. I 

 need only point out the strong curve of the umbonal ridge at its upper end and its 

 direction below for most of its length at nearly right angles to the hinge line. Other 

 points of varianiie will readily be seen by comparing the figures of the two species. 

 But perhaps the most similar form is M. swalloioi McChesnej^ which is a modioli- 

 form shell much resembling M. wyomm.gensis in shape, but with a sharper backward 

 sweep of the umbonal ridge. In Lea's species also the junction of the cardinal and 

 the posterior outline is angular, while in McChesney's form the posterior outline merges 

 imperceptibly into that of the cardinal margin. However, no comparison with 

 M. xwalJowi seems to be necessary, because there is evidence for believing that it is 

 genericallj^ distinct. It has a rather strong individual it}" of expression when com- 

 pared with other specimens of Myalina, and certainly seems to show structural differ- 

 ences which make its removal from de Koninck's genus desirable. Meek " was, I 

 believe, the first to call attention to this fact. Specimens from several localities 

 preserved as internal casts show that this form does not possess the massive striated 

 hinge plate which characterizes Myalina, but that, on the other hand, the cardinal 

 margin is thin and linear, with a long cartilage groove. It would seem, therefore, 

 that M. swallovji should be removed from Myalina. It can be referred, provi- 

 sionally at least, to Modiola. 



I hardlj' need compare with 2L hinsasensis., which, while it is somewhat similar, 

 lacks the interior lobe of Lea's species and is distinguished from all others known to 

 me by the fluted lamella? with which the sui-face is covered. 



The preceding description and comparisons are based upon material from the 

 Rico formation of the San Juan, region which I had prepared to publish as a new 

 species when fortunatelj^ there came into my hands the types of Myalina wyoming- 

 ensis^ which Lea described in 1853, from the Northern Anthracite field of Pennsylvania. 

 I find that my Rico fossils, which possibly hold about the same geologic horizon as 

 the Pennsylvania fauna, agree in the closest manner with the types of M. vyyoming- 

 ensh, and I can not consider their specific identity as doubtful. As I shall seek an 

 early opportunity to redescribe, refigure, and discuss from the type material all of 

 Lea's species whose characters have so long been problematical, I have not intro- 

 duced at this place typical figures for comparison. 



Myalina wyomingensis is extremely abundant at Scotch Creek (station 2340). 

 What I take to be the same form occurs also at Graham, Young County, Tex., and 



aU. S. Geol. Surv. Nebraska, etc., 1872, p. 202. 



