BRACHIOPODA. 415 



Plectambonites serlcea.l 



brachial process short, appressed and widely divergent; muscular impressions 

 [adductors] generally obscurely defined, occupying an obcordate area and separated 

 from each other by two subparallel, narrow ridges that sometimes coalesce near the 

 base of the cardinal process [and are strongly elevated and broadly thickened anteri- 

 orly] each impression usually nearly equally divided by a slender linear [sometimes 

 thickened and much elevated], straight ridge; anterior and lateral regions more or 

 less roughened by minute, granular, radiating strice. [These are the markings left 

 by the vascular and genital organs, anterior to which, in the thick shells, there is a 

 well developed ridge just inside the front margin.] 



"Ventral valve moderately convex, being nearly evenly, but gently, arched along 

 the middle from the beak to the front, and thus following so nearly the curve of the 

 other valve as to leave but a very thin visceral cavity within; beak very small, or 

 scarcely, if at all, distinct from the cardinal margin; arga twice to three times as high 

 as that of the other valve, inclined backward or more or less nearly parallel to the 

 plane of the valves; foramen arched over near the beak by a small deltidium, and 

 [nearly] closed between this and the hinge margin by the prominent cardinal process 

 [and chilidium] of the other valve. Interior showing hinge margin to be obscurely 

 [sometimes prominently] marked by minute pits for the reception of the crenulations 

 of that of the other valve; teeth small; [diductor] muscular impressions long, narrow, 

 separated behind by a short, linear, mesial ridge [upon each side of which are slender, 

 shallow depressions of the adductor muscles], and diverging and extending forward 

 beyond the middle of the valve, with a moderately distinct dental ridge along the lat- 

 eral margin of each; anterior and lateral regions granulo-striate." (Meek, op. tit.) 



Surface of both valves marked by numerous, very minute, closely arranged, 

 equal, radiating striae, or with every fourth, fifth or sixth one a little larger or more 

 prominent than those between. 



Plectambonites sericea varies considerably in size, convexity, outline and in the 

 strength of its muscular markings. The largest specimens observed were collected 

 in the lower portion of the Hudson River group near Granger, Minnesota, and one 

 of these is 28 mm. in width. Similar large examples occur at Cincinnati, Ohio, and 

 have received the name Leptcena aspera James. The crenulations along the lateral 

 margins of the hinge- line are a very marked character in specimens from the former 

 locality, and the muscular scars of the ventral valve are often not as divergent as in 

 others on the same slab. As a rule, shells from the Galena horizon are smaller than 

 those from the Hudson River or Trenton formations, to which Mr, Sardeson has 

 given the name P. minnesotensis. Specimens are usually abundant at most localities. 

 P. sericea is one of the few species extending through the Lower Silurian, and 

 is replaced in the Niagara by P, transversalis Wahlenberg. As stated in the 



