Trenton Gasteropoda of Manitoba. 325 
All the localities at which this species had been found, 
up to the close of 1889, are enumerated in the paper 
in which it was described. Since then it has been 
collected by Messrs. Dowling and Lambe in 1890 at 
Berens Island, at Sturgeon, Snake and Black Bear 
islands, Lake Winnipeg; by Mr. Lambe in 1890 at the 
Dog’s Head; and by Mr. Dowling in 1891 at Commis- 
sioners, Little Tamarack and Punk Islands, also at Grind- 
stone Point, Lake Winnipeg. It is one of the most abun- 
dant and characteristic fossils of the Trenton limestone of 
Manitoba, and according to Messrs. Weston, Tyrrell, Dow- 
ling and Lambe, it always occurs with the flat side upper- 
most in the rock. 
In 1890 Mr. Lambe collected an operculum, which is pro- 
bably that of a large specimen of this species at Jack 
Fish Island, Lake Winnipeg. This operculum, which is 
represented in outline in the wood cut on page 324, isa 
little more than four inches in height or depth, and not 
quite three inches in its maximum breadth. Its outer sur- 
face is completeiy buried in the matrix, the inner surface 
only being exposed.: In the woodcut, the side indicated by 
the letter A clearly corresponds to the outer side of the 
shell, and the concave side opposite,—B,—to the inner or 
columellar side. The side marked C corresponds to the ~ 
flattened spiral side of the shell, and that marked D to the 
inner wall of the umbilicus. The margins of the sides C 
and B, whose junction forms the ‘nuclear angle,” are 
thickened, but the edges of the other two sides are very 
thin. This thickening of the sides C and B is immedi- 
ately followed by a shallow depression in the nuclear 
region, but the inner side of the operculum is otherwise 
nearly flat. The surface markings of this side consist of 
numerous concentric raised lines of growth, but there are 
no clear indications of any “internal projections for the 
attachment of muscles.’’? Although the opercula of M. 
Logani, Salter, and M. crenulata, Billings, are known to be 
provided with well developed muscular processes on the 
inner side, this is by no means always the case in other 
