188 



Dimensions of one of the largest specimens known to the writer, the 

 cast of the interior of a right valve figured : length 12-2 mm.; height, 

 10 '3 mm.; estimated thickness of a cast of both valves, 5 mm. 



Casts of the interior or moulds of the exterior of one or both valves of 

 shells of this species are abundant at Inmost Island, Kinwow Bay, Lake 

 Winnipeg, where they were collected by T. C. Weston in 1884 and by D. 

 B. Dowling and L. M. Lambe in 1890. 



In the preceding description the shorter and narrower portion of each 

 valve is regarded as the anterior side, and the longer and broader as the 

 posterior, but it is by no means clear that these terms, as thus used, re- 

 present the true homologies of the shell. It may be that the shorter is 

 the posterior side and vice versa. 



This curious little shell is referred provisionally to the genus Edmondia, 

 at the suggestion of Mr. Ulrich, who, in a letter to the writer, dated 

 May 8th, 1896, make the following remarks upon specimens of it that 

 had been sent to him for examination and comparison : "After a most 

 careful investigation and comparison, I fail to find, despite my earnest 

 wish to do so, the remotest difference between this Manitoba Trenton shell 

 and the most typical Devonian and Carboniferous species of Edmondia I 

 The agreements are as follows : (1) the hinge is edentulous ; (2) there is an 

 internal plate in each valve beneath the beak, which extends posteriorly 

 and leaves in the cast (as your specimens show very clearly) a long nar- 

 row sharp slit or fissure running nearly parallel with and a short distance 

 within the cardinal margin ; (3) the muscular scars have the same position 

 and form, and the anterior one, which is vertically elongated, sends up 

 an obscure furrow to the front side of the beak ; (4) the surface of the 

 casts exhibit obscure traces of radiating lines ; and (5) the rounded form 

 and short anterior end. Considering these important agreements, to- 

 gether with the fact that the material at hand shows no differences, I do 

 not see that you can do otherwise than refer the species to Edmondia.'' 



In some manuals of paleontology or conchology Edm.ondia is said to 

 be restricted to the Permian and Carboniferous systems, but Hall * has 

 recently described and figured several species from the Devonian rocks of 

 the United States, and Zittel t says that it ranges in time from the 

 Silurian to the Dyas (Permian). 



GASTEROPODA. 



Tetbanota bidoesata, Hall. (Sp.) 



Bucania bidorsata, Hall 1847. Pal. N. York, vol. I., p. 186, pi. 40, 



figs. 2, a-g. 

 Tetranota bidorsata, Vlrich s,nd ScoSeld . . .1897. Geol. Minn., Final Rep., vol. III., 



pt. 2, p. 877, pi. 65, figs. 10-18. 



* In the PalEeontology of New York, vol. V., pt. 1, Lamellibranchiata 2. 

 t^Handbuch der Palaeontologie, vol. II., p. 128. 



