238 Annals of the Carnegie Museum. 



21. Plectorthis exfoliata Raymond. 



(Plate XXX\', figures ii, 12.) 

 Hsberiella exfoliata Raymond, 1905. American Journal of Science, series 4, Vol. 

 XX, p. 370. 



It is not easy to determine whether this species should be referred 

 to Hehertella or Plectorthis. The valves are nearly equally convex, 

 and the surface is marked by fine plications or coarse striae. The 

 statement in the original description that the sinus of the brachial 

 valve of this species is deeper than in Hehertella vulgaris'^ is incorrect, 

 two species being mixed up in the original types. Plectorthis exfoliata 

 is very abundant in the lower part of the Chazy at Chazy and Valcour, 

 but nearly all the specimens are exfoliated single valves. It is the species 

 usually identified as Orthis costalis. (See Hall, Paleontology New York, 

 Vol. I, PI. 4 bis, figs. 4&, 4?>, ^c\ and Hall and Clarke, ibidem, Vol. VIII, 



PI. 5, fig- 15.) 



Description. 



Valves nearly equally convex, the pedicle valve a little the deeper. 

 Outline variable, but usually somewhat transversely oval, the shells 

 being slightly wider than long. Hinge line nearly equal to the greatest 

 width. Cardinal area of pedicle valve moderately high, scarcely in- 

 curved. A single specimen shows a remnant of a deltidium. Pedicle 

 valve evenly convex, with a suggestion of a low median fold. Brachial 

 valve evenly convex or with a slight median sinus. Surface marked 

 by thirty to forty fine simple plications, two or three of which on each 

 valve seem to bifurcate. Specimens of this species are somewhat 

 similar to those specimens of Hehertella vulgaris which lack the sinus 

 in the pedicle valve, but the striae are much coarser. 



Locality. — This species is very common in the lower Chazy at Chazy, 

 Valcour, and Valcour Island, New York, and a few specimens have 

 been found at other horizons on Valcour Island. 



Genus Pl.esiomys Hall and Clarke. 



22. Plaesiomys platys (Billings). 



(Plate XXXV, figures 13, 14.) 

 Orthis platys Billings, 1859. Canadian Naturalist and Geologist, Vol. IV, p. 438, 



fig- 15- 



2 The Hehertella costalis of my papers in the American Journal of Science, Vol. 

 XX, and Annals Carnegie Museum, Vol. Ill, no. 4, should be read Hehertella 

 vulgaris. This mistake arose from a failure to restrict the Orthis costalis of Hall 

 to the specimen first figured by him. Some of the other types were Hebertellas. 



