236 H. W. SHIMER SPIR1FER0IDS OF LAKE MINNEWANKA SECTION 



Mexico. At Lake Minnewanka it is common in the transition beds be- 

 tween the Lower Banff shale and the Lower Banff limestone. 



Cyrtia cyrtinifornii* Hall and Whitfield 



This species is represented by numerous and quite well preserved speci- 

 mens. Their average size is much larger than the type from the Che- 

 mung at Roeki'ord. Iowa. The latter, an averaged sized specimen, has a 

 width of 11 millimeters : height of cardinal area of pedicle valve, 9 milli- 

 meters : length of brachial valve, 12 millimeters, as against 21. 12, and 

 19 millimeters (23 millimeters along the curvature) for a medium sized 

 imen from the Lake Minnewanka section. The pedicle valve of the 

 latter has a slightly more incurved cardinal area. The proportion- 

 the delthyrium are similar, as well a< the number of plications, on each 

 side of the fold and sinus — that is, about 20 — for here the plications are 

 simple: but as th se on fold and sinus bifurcate the number here has in- 

 creased on this enlarged shell to 20 at the anterior edge, resulting in 

 about 60 plications to a valve as against 45 to 50 in the Iowan forms. 



The mature shell is slightly incurved at the front, owing to a greater 

 development of the median sinus, though even in the young shell this is 

 more highly developed than in the Iowan forms. The young shell like- 

 wise is considerably wider than long. 



In size it is similar to the form noted by Whiteaves from the Hay 

 River, Mackenzie, but this has only 29 or 30 plications to each valve. 



Range. — Chemung of Iowa and Hay River. At Lake Minnewanka it 

 is very common in the transition beds from the Lower Banff shale to the 

 Lower Banff limestone, and likewise common in the uppermost portion 

 of the latter. 



MISSISSIPPIAX 



Irifer centrouatus Wmchell 



This is the most abundant spiriferoid of the Lake Minnewanka area. 

 It ranges through the en:::-: Mississippian series of beds, a thickness 



feet, and is represented in local areas throughout this series by 

 immense numbers of individuals. 



In its range from bottom to top of these strata, the species undergoes 

 certain variations, while in the lower portion of the Lower Banff shale it 

 is represented by the variety Mapinensis. It was during the deposition 

 of the upper portion of the Lower Banff shale that the species attained 

 its most typical form. 



