66 
The specimens identified with this species from the lower beds, section 
2-27 and 28, are much smaller than the ones in the higher beds. A shell 
of average size from 2-27 has a length of 15 mm. and a width of about 
20 mm. Accompanying this smaller size is a reduction of the lateral 
plications to five; in all other respects they appear to be identical. The 
young shell in the higher beds has six to eight lateral plications but those 
nearer the cardinal angles make their first appearance considerably later 
than the ones bordering the median sinus and fold. If our identifications 
are correct the most apparent change in the evolution of this species 
throughout the time represented by the deposition of 130 feet of limestone 
has been the great increase in size — the same kind of change noted in the 
evolution of so many other invertebrates, but more spectacularly so in 
the vertebrates. 
Abundant upon the northwest shores of lake Minnewanka, whence 
the name. 
Locality and Horizon. In the Minnewanka region in the Mississippian 
of sections 2-22 (c), 23 (c), 24 (c), 27 (r), 28 (c). 
Genus, Ambocoella Hall 
Ambocoelia magna n. sp. 
Plate III, figures 3 a, b, c; 4 a, b 
Description . Shell small, wider than long; cardinal extremities 
rounded. The dimensions of a pedicle valve are: length 15 mm. + , breadth 
20 mm., convexity 6 mm. Another pedicle valve of similar size has 
cardinal area 4 mm. high, delthyrium 4 mm. wide at base, and hinge-line 
about 10 mm. long. 
Pedicle valve very convex with greatest convexity posterior to middle, 
from which the surface descends in a straight line or with a slight concavity 
to the cardinal extremities, and anteriorly with a slight convexity. Beak 
rather narrow, pointed, slightly incurved. Cardinal area moderately 
high, arched, its lateral margins poorly defined. Delthyrium forming an 
equilateral triangle. A narrow median sinus extends from the beak to 
■the frontal margin, broadening slightly anteriorly; the bottom of the sinus 
is subangular, its sides undefined. 
Brachial valve unknown. 
The surface marked with faint concentric growth lines and a few 
growth ridges; the latter are especially noticeable towards the cardinal 
extremities. 
Remarks. This species appears to be most closely allied to A. union- 
ensis Weller from the Kinderhook (Rockford limestone) of Illinois, but 
an average specimen of that form measures 5 mm. long by 5 mm. wide; 
ours is thus about three times the size of that species. The Minnewanka 
species likewise has a proportionately longer hinge-line, a less concave 
slope of surface from umbo to cardinal extremities, and a somewhat less 
angular median sinus. 
Locality and Horizon . In the Minnewanka region in the Lower Mis- 
sissippian of section 2-35 (r). 
