144 NOTES ON THE FOSSILS OF ONTARIO. 



In many respects this singular species agrees with M. Canadensis, 

 Hall ; from which, however, it is clearly distinct. It is only known 

 to lis by the cast, which is entirely free from distortion, and may 

 therefore be relied upon as giving the true characters of the interior 

 of the shell. The cast forms an almost complete ellipse, which is 

 very much compressed laterally, and has a length of twenty-eight 

 lines, a width of nineteen lines, and a depth of seven lines. In 

 M. Canadensis, on the other hand, the depth of the valves is nearly 

 or quite equal to the width. Our species, therefore, entirely wants 

 the great ventricosity of the dorsal portion of the shell which so 

 distinguishes M. Canadensis. In the latter species the beaks are 

 enormously thickened, and the east exhibits a great conti-action or 

 excavation situated anteriorly above the casts of the musculai- 

 impressions. In M. compressus, on the contrary, the thickening of 

 the anterior portion of the shell must have been much less, and this 

 contraction of the cast is wanting. In both species alike, the ventral 

 portion of the cast is the thinnest, and a well marked shallow depresr- 

 sion or groove extends backwards from the muscular impression, 

 parallel with the margin of the shell and ultimately becoming obsolete 

 posteriorly. This indicates a corresponding ridge or elevation on tlie 

 interior of the shell. 



Locality and Formation. — Guelph Formation, Hespeler. 



98. MuRCHisoNiA BiviTTATA, Hall. Guelpli Formation, Hespeler. 



99. MuRCHisoNiA LoGANi, Hall. Guelph Formation, Hespeler 

 and Flora. 



100. MuECHisoNiA LONGISPIRA, Hall. Guelph Formation, Flora. 



101. Pleurotomaria Deiopeia, Billings. Guelph Formation, 

 Hespeler. 



102. HoLOPEA Gracia, Billings. Guelph Formation, Guelph. 



103. Strabarollus Hippolyta, Billings. Guelph Formation, 

 Hespeler. 



104. SuBULiTES VENTRicoSA, Hall. Guelph Formation, Hespeler. 



