33 
Remarks. Our single, partly exfoliated specimen belongs evidently to 
Phillip’s species as discussed recently by Weller. 1 The important external 
characters are the strength and direction of the concentric corrugations 
and the number of the radiating costae. These characters appear to be 
sufficient to separate the species from the earlier occurrences of Leptaena 
rhomboidalis , with which it is usually classified. The following comparison 
shows not only this difference, but likewise the apparent distinctness of 
the types of Leptaena rhomboidalis from the different periods. The speci- 
mens measured were from the Trenton of Anticosti island, the Niagara of 
Lockport, New York, the Onondaga of Waterville, New York, and our 
specimen from the Kinderhook of Minnewanka region. Several specimens 
were examined from each locality, except the last, and an average one 
taken for measurement, but the number of striae, character of corrugations, 
etc., were the same for all examples seen. The measurements are given in 
millimetres. The height of the corrugations given is the maximum, that of 
the one next the geniculation. 
— 
Trenton 
Niagara 
Onondaga 
Kinderhook 
Length 
12 
18 
21 
23 
Breadth at hinge-line 
20 
24 
30 
34 
Breadth midway of length 
18 
22 
24 
28 
Concentric corrugations 
low (0-2) 
higher 
narrowly 
rounded 
10-12 
high (1) 
ridge-like 
10-12 
high (1) 
ridge-like 
to rounded 
18 
Top of concentric corrugations 
rounded 
Radial strife in 5 mm 
23 
The Trenton and Niagaran forms have the lateral margins of the 
shell more nearly parallel than is the case with those from the higher beds. 
This is seen also in the development of those from the higher horizons; 
the young shell, as indicated likewise in the direction of the corrugations, 
has sub-parallel sides. Gradually the hinge-line is extended, at first with a 
sharp marginal concavity (typical of adult Devonian forms), later with a 
more gradual deflexion (typical of the adult Mississippian form). The 
concentric corrugations are very low, with rounded tops in the Trenton 
individuals, but become higher, with more narrowly rounded or ridge-like 
tops in shells from higher and higher beds. The number of radial costae 
appears in our specimens (18 occupy the space of 5 mm.) to be an important 
distinction; this changes from a large number in the Trenton form to few 
in Silurian and Devonian specimens, back to many again in the Mis- 
sissippian. 
Locality and Horizon. In the Lower Mississippian of North America. 
In the Minnewanka region in the Lower Mississippian of section 2-29 (R). 
Genus, Schuchertella Girty 
Remarks. This genus, otherwise quite similar, both internally and 
externally, to Streptorhynchus King, differs in its lower and wider cardinal 
area and in its undistorted umbo. ScheUwienella Thomas is otherwise 
similar internally, but has a flat or, usually, concave pedicle valve and a 
convex brachial valve. s&s 
1 Loc. dt. 
