223 



PLEUROMYA SDBCOMPRESSA, var. CABLOTTENSIS. 

 Plate 29, figs. 7 and la. 



Pleuromya Carlottensis, Whiteaves. 1876, This volume,p. 57, pi. 9, fig. 8. 



Myadtes subcompressus. White, (As of Meek.) 1880, United States Geological 



Survey. Contributions to Pale- 

 ontology, Nos. 2-8, pi. 38, fig. 5c; 

 cset. excl. 

 Comp. Pleuromya Newtoni, Whitfield. 1877. Preliminary Report on the 



Black Hills, p. 20. 



" " 1880, United States Geographical 



and Geological Survey of the 

 Rocky Mountain Region, Geology 

 of the Black Hills of Dakota, p 

 367, pi. 5, figs. 19 and 20. 



Umbones broad and depressed : beaks (or apices of the umbones) 

 curved inwards, downwards and inclined a little forwards : shape, with 

 this exception, and sculpture as in the type of P. subcompressa. 



South side of Alliford Bay, five casts of the interior : East end of 

 Maud Island, one specimen. On the mainland of British Columbia it 

 occurs also in the porphyrites and felsites of Sigutlat Lake and the 

 Iltasyouco Eiver, where it was collected by Dr. G. M. Dawson in 1876. 



The specimen from which the description of P. Carlottensis was 

 made is a distorted and imperfect cast, and the figure of it on Plate 9 

 of the present volume is by no means satisfactory. Although most of 

 the Canadian specimens collected since are also either distorted or 

 imperfect, they show that the shell is very variable in shape, and that 

 it is usually more elongated transversely than the original figure of it 

 might lead one to suppose. The anterior end, which is very short, is 

 excavated under the beaks and abruptly truncated below in the best 

 preserved examples : the posterior end is elongated and either narrowly 

 rounded or somewhat pointed at its junction with the ventral margin. 



Some individuals of P, Carlottensis appear to be intermediate in their 

 character between P. Newtoni, Whitfield, and P. subcompressa, Meek, 

 their shape being like that of the former species, and their sculpture 

 like that of the latter. Judging by the figure on Plate 38 of Dr. C. A. 

 White's " Contributions to Paleontology," other specimens appear to be 

 conspecific with the fossil from Devils' Slide, Cinnabar Mountain, 

 Montana, which Dr. White says " may prove to be a dfferent species," 

 but which he regards provisionally as '' only a variety of Myadtes 

 subcompressus." In the writers' judgment, P. Carlottensis also is doubt- 

 fully distinct from P. papyraced, Gfabb. 



March 27th, 1884. 3 



