DESCRIPTIONS OF SPECIES. 309 



Colorado which 1 have identified with M. arhansasana^ with this exception — that the 

 sinus above referred to does produce a slight emargination, giving the inferior 

 border a somewhat sinuate outline, which is, however, much less strongly marked 

 than in M. s^mllovji. 



While I have identified these Colorado shells with a certain degree of confidence 

 with a form occurring in the Genevieve division of the Mississippian Carboniferous, 

 the evidence thus afforded for correlation should not be overestimated. Modioloid 

 and aviculoid shells of this general type are common throughout upper Devonian 

 and Carboniferous time and are distinguished for their variabilitj-. Herrick figures 

 several closely similar shells from the Waverly group of Ohio, identified as Myalina 

 michiganensis (pi. -i, fig. 6) and Modiola loaverlyensls (pi. 4, fig. 10)," and others, 

 described or undescribed, doubtless occur.* The evidence of this species, therefore, 

 must be accepted with caution. 



Locality and hmnzon. — San Juan region (stations 2380, 2386, 238T); Ouray 

 limestone. Leadville district (stations 2372, 2377); Leadville limestone. Canyon 

 (station 2366)-, Millsap limestone. Pikes Peak quadrangle (station 2371); Millsap 

 limestone. Castle Rock quadrangle (station 2367); Millsap limestone. 



Myalina kegkuk Worthen. 

 PI. I, fig. 12. 



1875. Myalina keokuk. Worthen, Geol. Surv. Illinois, Kept., vol. 6, p. 524, pi. 30, fig. 5. 



Keokuk limestone: Keokuk, Iowa; Warsaw, Nauvoo, and Hamilton, 111. 

 1895. Myalina keokuk. Keyes, Missouri Geol. Surv., vol. 5, p. 117. (Date of imprint, 1894.) 



Keokuk limestone: Bonaparte, Iowa; St. Francisville, Mo. 



My material consists of a single specimen from the San Juan region (station 

 2382). It has something the shape of a parallelogram, having the anterolateral and 

 posterolateral margins nearly straight and parallel, and the superior margin, also 

 rectilinear, cutting them at an acute angle. The inferior margin is rounded. The 

 axis is strongly inclined to the cardinal line. The anterolateral margin is recurved 

 so that it is concealed beneath the shell when the latter is viewed from above. The 

 umbonal portion, though enlarging below, is thin, much elevated, and projecting. 



This species resembles 31. congeneris, M. sanctiludovici, and 31. Jceokuk, but 

 e.specially the latter, by reason of its elevated and projecting umbo. In size, 

 however, it is in accord rather with 3£. sanctiludovici, and as the three species men- 

 tioned above are very similar to one another, I am not altogether .satisfied that my 

 identification is the best one. 



Locality and horizon. — San Juan region (station 2382); Ouray limestone. 



"Sci. Lab. Denison Univ., Bull., vol. 3, 1888. 



&A distinct but evidently related species has recently been described by Weller under the name of Lilhophagaminuta. 

 (Acad. Sci. St. Louis, Trans., vol. 11, No. 9, p. 168. pi. 15, fig. 19). 



