248 Mr. T. R. Jones on North American 



IsocHiLiNA*. Subgenus of Leperditia. 



Equivalve ; the margins of the valves meeting uniformly, not 

 overlapping as in Leperditia', greatest convexity of the valves 

 either central or towards the anterior portion. Eye- tubercle 

 present. Muscular spot not distinct externally. 



4. Leperditia {Isochilina) Ottaiva, spec. nov. PL X. fig. 1. 



Length ,1, breadth y'^ inch. 



Leperditia-like in outline, somewhat elongate, smooth ; mar- 

 ginal border distinct, frequently seen to be marked by a line of 

 small, distinct pits ; eye-spot disthictly raised. 



From the Canal, Grenville. Gregarious ; the separated valves 

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

 limestone in the Calciferous Sandrock, a foot or two beneath the 

 " two-foot limestone," and traceable for some miles. 



5. Leperditia {Isochilina) gracilis-^, spec. nov. PI. X. fig. 2. 



Length |, breadth -^^ i^^^^- 



Carapace subrhomboidal, narrow and slender compared with 

 the LeperdititT proper ; anterior extremity obliquely rounded, with 

 the antero-dorsal angle produced, slightly obtuse ; posterior extre- 

 mity 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 32), which are 

 represented on the inside of the rim by corresponding raised 

 obtuse points. 



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 cry- 

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



Cytheropsis, genus, M'Coy. 



This generic appellation is affixed to a bivalved Entomostracan 

 (fig. 2. pi. 1 L) in the ' Systematic Description of the British 

 Palaeozoic Fossils in the Geological Museum of the University 

 of Cambridge,' 1855, but neither the characters of the genus 



* Equal-Up : Icros, equal ; x^'^^^- lip- 



t Referred to in Quart. .Tourn. Geol. Soc. vol. viii. p. 205. 



