DESCRIPTION OV SPECIES 237 



8. centronatus var. albapinensis Hall and Whitfield 



This form was described by Hall and Whitfield 4 as a species distinct 

 from 8. centronatus, and distinguished from it especially by the char- 

 acter of the median sinus. In albapinensis the median sinus is some- 

 times smooth, sometimes marked by two or three plications, which are 

 always faint and do not extend beyond the anterior half of the shell. 

 The sinus is bordered on each side by a broad, rounded plication, which 

 does not bifurcate as in S. centronatus, but continues prominent to the 

 anterior margin of the shell and undivided, except at times by a slightly 

 impressed line on the lower half. 



These are considered only varietal differences by Girty 5 and as repre- 

 sentative of the condition of most young shells. 



The Lake Minnewanka specimens of this variety are of very differing 

 sizes, even from the same bed. Thus in locality 35 (lower portion of the 

 Lower Banff shale), which is especially characterized by this type of 

 Spirifer, an exceptionally small specimen is 6 millimeters long by 9 milli- 

 meters broad, with about 12 plications on each side of the sinus, while a 

 specimen somewhat larger than the average measures 15 by 28 milli- 

 meters, with about 18 plications on each side of the sinus. All the forms 

 agree, however, (a) in the great breadth and prominence of the plica- 

 tions bounding the sinus and in their undivided character, except for a 

 slight flattening which at times develops into a faint mesial depression ; 

 (b) in the absence of plications in the sinus and in its peculiarly broad, 

 low flattened appearance; (c) in the appearance of the median fold, 

 which is quite prominently elevated and expands rapidly from the beak 

 to the front of the shell. It is impressed by a median line, and each of 

 its two halves thus formed is again in many cases divided by a shallow 

 sinus which extends up only a short distance from the anterior margin. 



The typical S. centronatus appears first in great abundance in the 

 upper portion of the Lower Banff shale. As there represented, it agrees 

 exactly with Girty's description and figures of the species as found in the 

 Yellowstone Park and in Nevada and Utah. 6 



The undivided bounding plications of the sinus of albapinensis have 

 here bifurcated near the beak, so that they lose their prominence and 

 approach the lateral plications in size. The sinus is characterized by six 



* Geology of the 40th parallel, vol. 4, p. 255. 



"Geology of the Yellowstone National Park. U. S. Geological Survey Monograph .'*-, 

 p. 548. 



•Geology of the Yellowstone National Park. U. S. Geological Survey Monograph ;rj, 

 part 2, p. 547. 



