I 
18 MUSEUM BULLETIN No. 31. 
the fold are not uncommon, and such forms arise by the bifurcation 
of one of the outer plications on the fold and the implantation of a 
corresponding plication in the sinus. The specimens with the extra 
plications are usually, though not always, larger than the average. 
Specimens from the Prasopora beds at Trenton, Out., usually show 
five plications in the sinus and six on the fold, though individuals 
with the normal number (%) are present. The number on each side 
of the fold and sinus is variable. There may be from seven to ten 
on the brachial and from eight to eleven on the pedicle valve; nine 
on one valve and ten on the other seems the most common number. 
The interior, of one of these shells is rarely seen. A pedicle valve 
from Trenton Falls, MY., shows a very small and short muscle scar, 
which is not at all thickened. Another from Peterborough, Out., 
shows a scar twice as long, much thickened at the front. The age of 
the individual has, of course, a great deal to do with the thickening 
of the scar, and it is probable that the specimen figured represents 
the scar of the normal adult of the species (Plate YI, figure 3). 
A good-sized adult is 19 mm. wide at the hinge, 12 mm. long, and 
10 mm. thick. Another is Vt mm. wide, 11-5 mm. long, and 9 mm. 
thick. An old individual is 15 mm. wide, 12-5 mm. long, and 12 mm. 
thick. All the above are from Trenton Falls, MY., and are in the 
Museum of Comparative Zoology, Cambridge, Mass. A specimen from 
Trenton, Ont., is 20 mm. wide, and 13 mm. long. One from the Oo- 
bourg beds at Picton, Ont., is 19 mm. wide, 12 mm. long, and 10 
mm. thick. 
Foerste has described another Trenton form of Platystropkia 
(P. colbiensis) from the Cynthiana formation of Kentucky. From his 
description and figures it appears that P. colbiensis differs from P. 
amcena in having the hinge width always less than the greatest 
width, in having fewer plications at the sides of the fold and sinus 
(8-9), and in 'being somewhat longer. Dr. Foerste has separated as 
varieties, specimens having the greatest width at the hinge and indi- 
viduals with extra plications in the fold and sinus. The shells of 
both varieties are larger and longer than in P. colbiensis , and none 
is so transverse as the shells from Canada and Mew York. Certain 
specimens from Trenton Falls may perhaps be identified as P. 
colbiensis , but the great width at the hinge differentiates most of 
them. 
Many small Platystrophias are found in the Eden at Cincinnati, 
but they are almost uniformly characterized by higher and more 
convex folds, and stronger plications than those in the Trenton and 
usually there are fewer plications at the sides. It would, however, 
undoubtedly be possible to pick out from the almost endless varieties 
