223 



Pleuromya subcomphessa, var. Carlottensis. 

 Plate 29, figs. 7 and 7a. 



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



Myacites subcoiiipress^is. Vihiie, {As of 'Meek.) — 1880, United States Geological 



Survey. Contributions to Pale- 

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

 cset. excl. 

 Comp. Plniromya Newloni, 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 nmbones) 

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

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



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 ofP, C'arZoi^ens/s appear to be intermediate in their 

 character between P. Neivtoni, 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 toPaheontology," other specimens ajjpear to be 

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

 Montana, which Dr. Wbite says "may prove to be a dfierent species," 

 but which he regai-ds provisionally as " only a variety of Myacites 

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

 fully distinct from P. papyracea, Gabb. 



March 27th, 1884. 3 



