EARLY SILURIAN ROCKS IN HUDSON BAY REGION 335 
and anterior portions of the valve. Similar variations occur in 
the shells of this species found at the Grand Rapids of the Sas- 
katchewan River, described by Kindle* as follows: 
Conchidium decussatum belongs to a group of shells in which the specific 
characters are very plastic. .... The ventral valve shows three well-marked 
types of contour, viz.: (1) Strongly convex with a more or less clearly defined 
median ridge extending from the umbonal region to the front of the shell. 
(2) Very convex with tumid umbonal region rounding regularly from the 
median region to the lateral and anterior margins without trace of median 
ridge. (3) Strongly convex in median and anterior region with or without 
median ridge, but with a broad shallow sinus extending from the beak across 
_ the umbonal region. These three types of contour make striking contrasts 
when individuals in which they are best developed are compared; but the 
intermediate forms, in which neither the presence of ridge or sinus nor their 
entire absence can be positively stated, make difficult any attempt to dis- 
criminate them as distinct varieties. 
It is noteworthy that the shells showing a mesial sinus in the 
umbonal region of the ventral valve are young forms, and the 
writer is convinced that the more striking differences shown in 
the ventral valve of this species represent different growth stages in 
the individuals; the youthful stages show a mesial sinus from the 
beak across the umbonal region or farther anteriorly, while in the 
old stages the mesial sinus has disappeared and a distinct mesial 
fold is frequently developed. . 
The above-mentioned characters are the principal ones on 
which Twenhofel founded the genus Vzrgzana, and it seems cer- 
tain that the species formerly known as Conchidium decussatum 
really belongs to the genus Virgiana. Through the kindness of 
Dr. Kindle a comparison was made of the shells of this species from 
Nelson River with those from the Saskatchewan region in order 
to make sure of the identity of the species from the two localities. 
The shells from the Grand Rapids locality also show unmistakably 
the characters of the genus Virgiana’? to which this species is here 
referred. 
« E. M. Kindle, ‘‘ Notes on the Geology and Paleontology of the Lower Saskatche- 
wan River Valley,’ Geol. Surv. of Canada, Mus. Bull. No. 21 (Geol. Series No. 30), 
October 14, 1915, p. 10. 
2Specimens of these shells were also sent to Dr. Twenhofel, the author of the 
genus, who agreed with the writer that they were true Virgianas. 
