24 
MUSEUM BULLETIN No. 31. 
' Interiors unknown. 
Length of the holotype, 16 mm., width 23 mm, 
A surprising number of relatively rare species of Strophomena 
occur in the Trenton, but this form does not fit in with any previously 
described. It belongs to the group of short shells whose length is 
about two-thirds the width, which at once precludes comparison with 
Strophomena filitexta Hall (Plate VII, figure 3), S. conradi Hall and 
Clarke, S. thalia Billings, S. trilohata (Owen), and S. winchelli Hall 
and Clarke (Plate VII, figure 1). There still remain, however, seven 
species with which it may be compared, 
Strophomena hillingsi Winchell and Schuchert, Strophomena 
trentonensis , and S. emaciata of the same authors have s.omewhat the 
outline of the present species but are much flatter shells and S. 
foveata lacks the wrinkles of the second species. Strophomena 
higginsportensis 1 Foerste is likewise described as being an almost flat 
shell, seemingly very like S. trentonensis . 
Strophomena scofieldi Winchell and 'Schuchert has a fold and 
sinus, and both S , septata Winchell and Schuchert and S. vicina 
Foerste are larger shells. 
Horizon and Locality. Rare in the Prasopora zone at Peter- 
borough, Ont., where the type was collected by E. J, Whittaker and 
fhe writer. 
Genus, Plectambonites Pander. 
Plectamhonites youngi sp. nov. 
Plate VII, figures 7, 8, 
A large Plectamhonites , concave-convex, the brachial valve 
following closely the contour of its opposite. Pedicle valve strongly 
convex, the curvature rendered irregular by one or more low, longi- 
tudinal ridges which cross the shell on or near the median line. 
Some of the shells show little or no trace of these ridges. The greatest 
width is at the hinge, which is sufficiently extended to produce small 
rounded “ears.” The length is two-thirds the width, a proportion 
which holds good for both the large and the email shells in the col- 
lection. 
iSurface marked, as is usual in the genus, by fine, tbread-like 
stride of two sizes which cover the surface densely. The major striae, 
from 30 to 35 in number, are irregularly spaced, so that between any 
pair there may be from three to six of the smaller etriations, but 
i What may we ultimately come to in the choice of specific names I 
