98 CANADIAN FOSSILS. 



From the caual at Grenville. Gregarious ; the separated valves 

 forming a thin layer about half an inch thick, in a dark-grey lime- 

 stone of the Calciferous Sandrock, a foot or two beneath the " two- 

 foot limestone," and traceable for some miles. 



In several specimens of Leperditia rock from the Chazy lime- 

 stone near the N.W. corner of the township of L'Orignal, I have 

 recognized the Isochilina Ottawa, under similar conditions to those in 

 which it occurs at the Grenville canal, except that in one specimen 

 it is associated with a Modiolopsis-Wke shell. I have only to remark, 

 that when the shell is broken off, the casts of the valves show a 

 distinct muscle-spot (concave on the inner side of the valve) with 

 numerous radii. 



Collectorsi — Sir W. E. Logan, J. Richardson, R. Bell. 



VI. Leperditia (Isochilina) gracilis, Jones. 

 Plate XL Fig. 15. 



{Annals of Natural History^ 3d series, vol. i. p. 248, plate x. fig. 2.) 



Length 4, breadth J^ inch. 



Carapace sub-rhomboidal, narrow and slender when compared with 

 the Leperditice proper ; anterior extremity obliquely rounded, with the 

 antero-dorsal angle produced, slightly obtuse; posterior extremity 

 rounded, with the postero- dorsal angle obliquely truncate. Ventral 

 curve uniform. Surface of valve convex centrally, black, shining, 

 smooth, sparsely punctate ; the pitting partial often obscure, or 

 nearly obsolete. Depressed margin broad, in many specimens bearing 

 a row of rounded pits (about thirty-two), which are represented on 

 the inside of the rim by corresponding raised obtuse points. 



Locality and Formation. — Gregarious ; in loose fragments of a black, 

 fine-grained foetid limestone from the White Horse Rapids, Isle 

 Jesus, referred, with doubt, to the Trenton limestone in the Quart. 

 Journ. Geol. Soc, vol. viii. p. 205, but to the Birdseye limestone in 

 a letter of later date from Sir W. E. Logan. The disunited valves 

 lie matted together, and sprinkled with minute iridescent crystals 

 of pyrites, in a thin layer or layers in the rock. 



Collector. — Sir W. E. Logan. 



Genm Cytheropsis, M'Coy. 



This generic appellation is affixed to a bivalved Entomostracan 

 (figure 2, plate 1. L.) in the * Systematic Description of the British 



