ORDOVICIAN FAUNAS. 159 
RHYNCHOTREMA INAEQUIVALVIS (Castel.). 
Plate X., Figs. 15-18. 
1895. Rhynchotrema inequivalvis W. & 8., Pal. Minn., pt. L., p. 459, 
pl. 34, figs. 9-25. 
Deseription.—Shell varying from narrowly to broadly subtriangular 
in outline. Each valve marked by from sixteen to twenty-two simple, 
prominent, subangular plications, with from three to five on the fold 
and two to four in the sinus. Pedicle valve strongly convex in the 
umbonal region, sloping more or less abruptly laterally, often angular 
near the anterior margin: mesial sinus originating on the umbo, often 
profound anteriorly, with abrupt sides. Beak more or less incurved, 
and always elevated beyond the umbo of the dorsal valve, with a 
narrow dethyrium, partially closed by deltidial plates. Brachial valve 
more convex than the other, mesial fold beginning at the apex of the 
shell as a flattening or slight depression, more or less strongly elevated 
anteriorly. Beak projecting into the delthyrium of the pedicle valve. 
The dimensions of an average specimen are: length, 10 mm.; width, 
11 mm.; thickness, 6.5 mm. 
Remarks.—This species is a common one in some localities of the 
Trenton limestone of New Jersey in the lower half of the formation, 
but it is never preserved so as to show the more delicate surface char- 
acters. On well-preserved specimens from some other regions the 
shell is marked by conspicuous, sublamellose, concentric growth lines, 
which are crowded near the anterior margin of the shell, but ‘these 
are not exhibited on any New Jersey specimens which have come 
under observation. 
RHYNCHOTREMA DENTATA (Hall). 
Plate X., Figs. 19-22. 
184%. Atrypa dentata Hall, Pal. N. Y., vol. I., p. 148, pl. 33, figs. 
WA ac: 
Description.—Shell small, transversely subelliptical in outline, the 
proportions of length to breadth being about as five to six; obtusely 
angular at the beak. Hach valve marked by seven or eight simple, 
