Palaozoic Bivalved Entomustraca. 245 



(thickness), which is usually greatest at the antero-ventral third. 

 Surface smooth. Eye-tubercle generally well marked, and 

 muscle-spot often distinct ; but occasionally the latter becomes 

 involved m the nuchal depression, and the former is sometimes 

 obsolete. 



This is the smallest form of Leperditia which I have yet met 

 with. It occurs in great numbers, together with Beyrichia 

 Logani in equal abundance, in a dark-grey friable limestone, 

 mainly composed of these Entomostraca, fragments of Trilobites, 

 and shells, at Grenville and near Hamiltonville in Hawkesbury, 

 on the Ottawa. This Leperditia-limestone forms part of a band 

 of limestone, about 2 feet thick, which extends over a wide di- 

 strict*, and is of importance as marking the position of a con- 

 tinuous band of concretionary phosphatic rock which is beneath 

 it, and belongs cither to the base of the Chazy limestone f; or 

 the summit of the Calciferous Sandrock]:. 



L. Canadensis occurs also in a dark-grey, crystalline, shelly 

 limestone (of the Calciferous Sandrock) at Grande Isle § (north 

 side), in the St. Lawrence. In two hand-specimens of this lime- 

 stone a inyt separate valves and one pair of valves are present. 



a. Var. lubrusa. PI. IX. fig. 13. 



Length ^j breadth -^-q inch. 



The extremities of the valves are in this specimen from Hawkes- 

 bury marked by a broad marginal depression, which is continued 

 less strongly along the ventral border; and the antero-dorsal 

 corner is more produced than usual. 



This may be an individual modified by accidental circum- 

 stances of growth. 



b. {Leperditia Canadensis t PI. IX. figs. 16, 17.) 



Specimens of possibly the same species as the foregoing, but 

 of a considerably larger size (often twice as large), occur in two 

 other Timestones, specimens of which Sir W. Logan has confided 

 to my care. 



Imbedded in bits of black fine-grained limestone from Louck's 



* "• This rock, having hcen quarried for lime-burning in several places, 

 has been followed from Carillon to Grenville (thirteen miles)." Quart. 

 Journ. Geol. Soc. vol. viii. p. 207 j and Logan's Report Geol. Surv. Ca- 

 nada, 1851-52, ]). IS. 



t The Atrypa plena, which is characteristic of the base of the Chazy 

 limestone, appears to occur above the Lcperditia-bed. 



X Esquisse gt'oloi;ique du Canada, jjar W. E. Logan et T. S. Uunt, p. 42. 



§ Quart. Journ. Geol. Soc. vol. viii.p.2<^2 ; and Logan's Report, 1851-52, 

 p. l.i. 



