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 striae. [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; area 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. cit.) 
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 aspern 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 
