FAUNA OF THE TRENTON GROUP. 
17 
brachial shell. Young specimens show no trace of this, but it is feebly 
developed on adults (shells 13 mm. long). The striations are prac- 
tically the same as in D. rogata. 
Young specimens can be distinguished from D. r ogata by the 
shorter hinge, and absence of the sinus in the brachial valve and 
ridge on the pedicle valve. 
The holotype 19 16 mm. long, 18> mm. broad, and the width at the 
hinge is 9 mm. The paratype is 10 mm. long, 10-25 mm. wide, and 
the width at the hinge is 5 mm. 
This species agrees very well in outline with D. testudinaria 
(Dalman), but lacks the alternating strife of that species. 
Horizon and Locality. Found only at the north end of Grand 
island, Balaam lake, Victoria county, Ont., where the types were 
collected by E. J. Whittaker. The horizon of these beds is still in 
doubt but is probably upper Black River, and certainly below the 
Crinoid beds of the Trenton. It is associated with Dalman ella rogata. 
Genus, Platystrophia King. 
Platystrophia amcena McEwan. 
Plate VI, figures 1-5', and 11. 
Platystrophia amcena McEwan, Proc. TJ.S. Hat. Mus., vol. 56, 1919, 
p. 412, PL 43, figures 1-8. 
Platystrophia lynx and biforata ( partim ) of authors. 
Recent work on Platystrophia by McEwan has broken up the old 
“Platystrophia biforata. var. lynx” of America into a very large num- 
ber of named species. The one here referred to is that most fre- 
quently found in the Canadian Trenton. 
Description. Shell below the average size for the genus, the 
valves moderately and subequally convex, the plications not coarse, 
and, especially at the sides, rather sharp; those on the fold often more 
broadly rounded. The greatest width is at the hinge, the cardinal 
angles usually projecting a little even in old specimens. The sinus 
is shallow, flat-bottomed, or slightly concave, the fold very slightly 
elevated and with a gently convex or flat crest. The great majority 
of specimens have three plications in the sinus and four on the fold. 
The median plication in the sinus is often noticeably stronger than 
the others, and there is a corresponding median furrow on the fold 
separating the two pairs of plications. There may be as many as six 
plications in the sinus and seven on the fold, but such specimens are 
unusual. Specimens with four plications in the sinus and five on. 
10711—3 
