DESCEIPTIONS OF SPECIES. 421 



anterior to the posterior extremity of the umbonal ridge, from thence the basal 

 margin passes in a straight line to the beaks. 



"The anterior basal area is nearly straight, flat, broad, slightly gaping and very 

 much depressed, a great portion of the surface being almost on the same plane with 

 the umbonal ridges. 



"Surface marked by fine concentric lines of growth, which are barely perceptible 

 along the more prominent parts of the shell. 



"Length, 15 mm.; breadth, 5.5 mm; greatest thickness, 4 mm. 



"This species somewhat resembles M. perattenuata, M. & H. , but is readily 

 distinguished from that shell b}^ its more angular and prominent umbonal ridge, 

 flattened, depressed anterior basal area, and by being more attenuated. 



^^ Position and locality. — From a hard, dark micaceous sandy shale belonging to 

 the Upper Carboniferous series and probably equivalent to the Middle Coal Measures, 

 Ouray, Colo." 



With the tj'pes of both species before me the differences pointed out by Gurley 

 by which M. cimeiformis might be distinguished frotn 3f. perattenuata do not in the 

 main hold good, though for the present at least I shall retain his species as distinct. 

 If. jwrattemuita seems to have the umbonal ridge directed at a slightly less acute 

 angle with the hinge line. It is possibly i^roportionally more elongate and has the 

 posterior cardinal angle better marked. This portion of the shell has the appearance 

 of being truncated (if the present specimens are not imperfect). The shape of 

 M. 2)erattenuata, in other words, approaches a parallelogram, that of M. cuneiformis 

 a triangle. 



At station 2314 in the Crested Butte region were collected sevcal slabs of black 

 limestone abundantly covered with a small species of Myalina in fragments and entire 

 valves, which is closeljr related to M. cimeiformis Gurle^'. The geologic horizon 

 is also probably the same. Though a number of individuals were collected, all the 

 material is so badly weathered that only one or two specimens are at all available for 

 illustration, while the entire series affords but an unsatisfactory concept of the species. 



Since examining the types of Gurley's species I am somewhat more inclined to 

 doubt the correctness of the identification of these Crested Butte fossils than when 

 dependence was placed upon his description alone. I have not, however, altered the 

 identification priginally set down, but leave the decision as to its correctness to be 

 decided by more complete material. 



The configuration which my specimens oi'iginally possessed can at present only 

 be estimated, but so far as I can judge the angle between the umbonal ridge and the 

 hinge line is somewhat less acute than in the types. 



Locality and, horizon. — Originally described from the lowest exposed Carbonifer- 

 ous beds at Ouray, probably the same as station 2195. Crested Butte district (station 

 2314)^ Weber limestone. 



