10 



Length of the largest specimen known, sixty millimetres; maximum 

 height of the same, thirty mm. 



Durham, J. Townsend : five fine specimens with the test preserved. 

 Two casts of a shell which probably belong to this species were col- 

 lected at Hespeler by T. C. Weston in 1867. 



Megalomus compressus. (Nicholson & Hinde.) 



MegaZomus compressus, Nicholson & Hinde. Report on the Palaeontology of 

 Ontario, 1875 ; pp. 68, 69. 



The above species or varietal form was described and figured from 

 mere casts, but Mr. J. Townsend has recently collected fine specimens 

 of it at Durham, with most and in some cases the whole of the shell 

 beautifully preserved. The test of M. compressus is rather thick (about 

 five millimetres in thickness on the umbones) especially in the umbonal 

 region, and its outer surface is concentrically striated. 



Fig. 1. Megidomug compresms, Nicholson & Hinde. Outline of left valve of a 

 typical but possibly extreme variety, mth most of the test preserved. 



The character most relied upon as a means of distinguishino- J/. 

 compressus from M. Canadensis is the lateral compression of the valves of 

 the former. In M. compressus the thickness through the closed valves is 

 stated to be " more than one third of their maximum height," whereas 

 in M. Canadensis the convexity of the shell is about equal to its great- 



