27 
This increase in the number of vesicles, with a corresponding decrease 
in their size, is a normal accompaniment of old age in corals, either of the 
individual or of the species. There is apparently a decrease in the ability 
of the animal to withdraw much of its lime-secreting basal tissue at one 
time. We would thus naturally expect to find the species becoming mul- 
tivesiculate in the higher beds. Such slight differences would hardly be 
worth considering were it not that they help us to distinguish between the 
representatives of a single species in two separate periods. 
Locality and Horizon. Mississippian and Pennsylvanian of western 
North America. In the Minnewanka region in the Pennsylvanian of 
sections 1-32 (C); 2-6 (C), 8 (C). 
Lithostrotion pennsylvanicum n. sp. 
Plate V, figures 3, 4, 5 
Description. Corallum compound, massive, rapidly expanding hori- 
zontally. One almost complete specimen has a diameter of 230 mm. and a 
height of 40 mm. Another specimen has a diameter of 150 mm. (incom- 
plete) and a height of 80 mm. (apparently almost complete). As seen from 
the above, the corallites diverge rather rapidly. 
Corallites crowded, producing polygonal individuals; varying greatly 
in diameter in the same colony, depending upon the age of the corallite. 
In one head 10 inches in diameter, individuals were noted varying in 
diameter from 2 mm. to 16 mm. ; the average diameter of a mature coral- 
lite is, however, from 10 mm. to 12 mm. 
Calyx very shallow, except in the central half which is suddenly, 
usually vertically, depressed into a cup from 5 to 6 mm. wide and 4 or 5 
mm. deep. Columella rather small, seldom laterally compressed, rising 
rather suddenly nearly or quite as high as the sides of the cup. Seldom, 
and then only in the smaller corallites, does the columella rise from the 
broader projecting top of the inverted, funnel-shaped tabulae. 
Principal septa 20 to 25 in number and extending to the columella; 
alternating with these is a shorter series, making a total of 40 to 50 septa. 
Tabulae numerous, bending downward conspicuously at almost a right 
angle around the columella, then arching outward and downward more 
gently to meet the upward bending dissepiments in an acute angle. The 
dissepiments at first bend upward vertically, then gradually arch outwards. 
They meet the walls of the corallite, in a mature individual, usually at an 
angle of 70 degrees to 100 degrees, at times 140 degrees, seldom as low as 
50 degrees; in young individuals up to a diameter of 5 mm. the angle is 
usually 50 degrees. Thus in the growth of the corallite the dissepiments 
at first meet the walls at a sharp, upward pointing angle; with increase of 
width, however, the tabulae arch more and more outward and downward. 
Dissepiments numerous in outer zone, absent within; the junction of these 
two zones is a weak spot and weathered specimens frequently show this 
inner part (8 mm. wide in both a 12 mm. and an 18 mm. corallite) alone 
remaining. 
