550 GEOLOGY OF THE YELLOWSTONE NATIONAL PAKK. 



biplicatus var. semifurcatus, S. centronatus has somewhat smaller and more 



numerous plications, and the fold, which is perhaps not quite so elevated, 



is surmounted by four or more equal radii. 



Formation and locality: Madison limestone, Hunter Peak, Absaroka 



Range ; T. A. Jaggar. Near summit of ridge, west end of Hunter Peak, 



Absaroka Range ; Arnold Hague. Crowfoot Ridge, Gallatin Range, top of 



bed 26 ; J. P. Iddings and W. H. Weed. Under Quartzite Ridge, north side 



of Burnt Pork. 



Spirifer subattenuatus Hall. 



Spirifer subattenuatus (by mistake, Spirifer submucronata) Hall, 1858: Geol. Surv. 



Iowa, Vol. I, Pt. II, p. 501, PI. IV, figs. 3a-3c. 

 Spirifer subattenuata Winchell (A.), 1862: Proe. Acad. Nat. Sci., Philadelphia, 



p. 4C5. Whiteaves, 1891: Contributions to Canadian Palaeontology, Vol. I, 



p. 223. 



Shell rather small, semicircular, alar angles somewhat rounded to sharp. 

 On either side of the fold and sinus there are eight to ten comparatively 

 large radiating ribs, which are crossed by strong concentric imbricating 

 ridges. Very variable in the number of plications on fold and sinus. One 

 dorsal valve shows a low fold with two strong radii equal to those on the 

 wings. Another specimen has a rather high fold with a very faint median 

 furrow. One ventral valve, on the other hand, shows a sharp median ridge 

 in the rather deep sinus, while still another has the sinus simple for over 

 half its length, when suddenly four low plications, somewhat smaller than 

 the rest, appear in it. Length of this last specimen, 10.5 mm.; width, 15 

 mm. Another example is considerably larger than this. 



The form in question seems to be closely allied to S. centronatus, from 

 some varieties of which it is not widely separated. It also bears some 

 resemblance to 5'. strigosus Meek and S. argentarius Meek, to the latter in 

 point of size, to the former in the plications of fold and sinus. 5'. argentarius 

 and S. strigosus are both described from Devonian strata; so is S. subattenu- 

 atus, but it is known to occur in the Waverly also. The material studied 

 is unsatisfactory. 



Formation and locality: Madison limestone, Stmkingwater Valley, 

 below mouth of the canyon, Absaroka Range; Arnold Hague. Chemung 

 age, Independence and Buffalo, Iowa: Rock Island, Illinois; Naples, New 

 York; Athabasca River, Canada. Marshall group, Point aux Barques, 

 Michigan. 



