PalcBOZoic Bivalved Entomostraca. 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 in the nuchal depression, and the former is sometimes 

 obsolete. 



This is the smallest form of Leper ditia 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 either to the base of the Chazy limestone f, or 

 the summit of the Calciferous SandrockJ. 



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 few separate valves and one pair of valves are present. 



a. Var. labrosa. PL IX. fig. 13. 



Length ^, breadth ^V 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. [Leper ditia 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 limestones, 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 been quarried for lime-burning in several places, 

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

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

 nada, 1851-52, p. 18. 



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

 limestone, appears to occur above the Leperditia-bed. 



X Esquisse ge'ologique du Canada, par W. E. Logan et T. S. Hunt, p. 42. 



§ Quart. Journ. Geol. Soc. vol. viii. p. 202 ; and Logan's Report, 1851-52, 

 p. 15. 



