BRACHIOPODA. 413 
Plectambon i t es . 1 
rhomboidalis, which so persistently recurs with more or less numerical strength 
throughout all formations from the Trenton of New York to the base of the Lower 
Carboniferous. 
In Anticosti Strophomena nitens Billings* occurs, which, as far as external 
characters are concerned, appears to be identical with specimens from Wil- 
mington, Illinois, examined by one of the writers. The interiors of these show 
them to be a species of Leptcena Dalman, and they are apparently closely related 
to L. unicostata. 
Formation and ZocaZ%.— Abundant in the upper portion of the Hudson River ^roup at Spring 
Valley, and rare in the lower portion of the same formation at Granger, Minnesota. Common at Graf, 
Iowa; Iron Ridge and Delafleld, Wisconsin; Savannah and Wilmington, Illinois. 
Collectors.— W. H. Scofield, E. O. Ulrich and the writers. Also in the collection of Dr. C. H. 
Robbins, Wykoff, Minnesota. 
Mus. Reg. Nos. 275, 8138-8141. 
Genus PLECTAMBONITES, Pander. 
1830. Plectambonites, Pander. Beitrage zur Geognosie des russischen Reiches, p. 90, pi. iii, 
tigs. 8, 16; pi. xxvm, fig. 19. 
Leptoena of authors, not Dalman. 
1892. Plectambonites, Hall. Palaeontology of New York, vol. viii, pt. i, p. 295. 
Description: "Shells usually small, normally concavo-convex. Surface covered 
with very fine striae, often alternating in size. Hinge-line making the greatest width 
of the shell, the extremities often subauriculate. Cardinal area narrow in both valves, 
crenulated on the margins. On the pedicle valve there is a moderately broad del- 
thyrium partly closed by a convex plate, but mostly occupied by the cardinal process 
of the opposite valve. Apical foramen sometimes retained. Teeth prominent and 
supported by thickened plates, which are continued in broad outward curves for 
more than half the length of the valve, returning and uniting in the umbonal cavity, 
thus limiting two linguiform [diductor] muscular scars, enclosing a more or less 
clearly defined adductor impression. 
"In the brachial valve the dental sockets are deep and often appear to transect 
the cardinal area. The cardinal process is simple and erect, but by its coalescence 
with the short, prominent, crural plates the posterior face appears trilobate. The 
crural plates end abruptly as in Orthothetes, becoming thickened at about the middle of 
their length and giving origin to two low ridges or septa, which at first approach each 
other and thence continue forward with a slight divergence, thus forming the inner 
boundaries for two elongate [adductor] muscular scars, which are less sharply defined 
in their outer margins. The muscular area is rendered quadripartite by two short 
transverse or oblique posterior furrows. Vascular impressions radial, sometimes 
digitate. Shell substance fibrous, sparsely punctate. 
*Pal. Foss., vol. i. p. 118, fi^. 07, 181)2; ('au!i,fli;ui Nat. aiul Geol., vol. v, p. 5:i. tis?. 1, im). 
