BEACHIOPODA. 413 



Plectambonites. 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 locality. Abundant in the upper portion of the Hudson Kiver group 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. Scofleld, E. O. Ulrich and the writers. Also in the collection of Dr. C. H. 

 Robbins, Wykoff, Minnesota. 



Mus. Beg. Nos. 275, 8138-8141. 



Genus PLECTAMBONITES, Pander. 



1830. Plectambonites, PANDER. Beitritge zur Geognosie des russischen Reiches, p. 90, pi. in, 



tigs. 8, 16; pi. xxvm, flg. 19. 

 Leptcena of authors, not DALBIAN. 

 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. 97, 1882; Canadian Nal. and Geol., vol. v. p. 53. tig. J, 1880, 



