Fossils of the Chazy Limestone. 439 



Dorsal valve gently convex, and with sometimes a barely per- 

 ceptible mesial depression. Surface of both valves marked with 

 fine radiating ridges, which bifurcate once or twice before reaching 

 the margin ; from eight to ten ridges in two lines of the width of 

 the front. Width about one inch ; length about nine lines. 



This species somewhat resembles 0. subquadrata (Hall), but is 

 always flatter and more finely striated. It occurs in great abund- 

 ance in the Chazy limestone, but is generally exfoliated, so that 

 good specimens are rare. 



Formation and Locality. — Chazy limestone, island of Montreal. 

 Several specimens from Wolfe Island, near Kingston, appear to 

 be referable to this species. 



Collectors. — Sir W. E. Logan, J. Richardson, E. Billings. 



OllTHIS PORCTA, (N. S.) 



Figs. 16, 17, 18. 



Figs. 16, 17, 18. — Orthis Porcia. Fig. 18 is a portion of the surface 

 enlarged to shew the concentric imbricating striae. 



Description, — Ventral valve conical, semi-oval ; width a little 

 greater than the length ; hinge-line about equal to the width of 

 the shell ; area large, much elevated, at right angles to the 

 plaue of the margin, very slightly curved, beak erect and pointed ; 

 foramen narrow, extending to the beak; surface with thirty sharp 

 elevated radiating ridges, with shorter ones between each two at 

 the margin, crossed by fine, strongly imbricated concentric striae 

 which, although fine, give the surface a rugose appearance, 

 Width four lines ; length three lines ; height of area from hinge- 

 line to beak one line and a half ; width of foramen at hinge-line 

 half a line. Dorsal valve unknown. 



Only a single valve of this beautiful little Orthis has been col- 

 lected, and I am inclined to think therefore that it must be an 

 exceedingly rare species. The form and size are very nearly those 

 of 0. disparalis, but it differs from that species in having the 

 radiating ridges not so strong and in the more perependicular area 

 and imbricating concentric striae. 



Locality and Formation. — Two miles north of city of Montreal. 

 Chazy limestone. 



Collector. — E. Billings. 



