108 (Hncimiati. Societi) of JS/'atural Hisforii. 



It differs from the dorsal valve, iu that it possesses this aperture, is 

 a little less convex, the apex a little more depressed, the imbricating 

 lines a little less distinct, and numbering only four or five instead of 

 six or seven. 



The following is the definition of the genus by Prof. Hall, founded 

 upon the dorsal valve: 



"Shells small, patelliform; apex sub-central, excentric or terminal. 

 Surface marked by concentric lamellaj of growth, which are more 

 expanded on the posterior side. Interior a shallow, oval cavity, with 

 bilobed muscular impressions; the margin flattened or slightly de- 

 flected, and entii'e." 



Twelve species have been described, ranging from the Trenton to 

 the Hamilton Group, as follows : One from the Trenton, one from the 

 Hudson River, one from the Cincinnati, two from the Niagara, one 

 from the Lower Helderberg, two from the Oriskany, one from the 

 Schoharie, and three from the Hamilton Group. 



When Prof. Hall established the genus, he regarded these fossils as 

 reall}^ univalve, like Patella. He said, however, in Paleontology of 

 New York, Vol. III., p. 489, that it is not easy to determine whether 

 these small shells are bivalve brachiopods, or univalve, like Vapulus. 

 Later, in 1867, Paloiontology of New York, Vol. IV., pp. 31, 

 413, Plate III., he showed that these fossils are bivalve brachiopods, 

 but he had found no evidence of a foramen in either valve. Our 

 specimens clearly show an aperture for a minute pedicle fibre of at- 

 tachment, and prove that the substance of the shell of the ventral valve 

 did not attach to any other object. We think this fact should remove 

 the fossil from the famil}^ Craniidm, but on account of the calcareous 

 character of the shell, we are not prepared to place it in the Discinida',- 



ER RA TA: 

 Page 95, 3d line from bottom, for Protaster, read Protastekina. 

 P.age 102, 12th line from bottom, for J. W. Harris, read I. W. Harris. 



