258 TRANSACTIONS OF THE [maR. 15, 



ent near the anterior margin, being intercalated next the outer 

 boundaries of the sinus. Each side of the valve, outside the 

 sinus, ornamented with from six to twelve rounded plications, of 

 which the one bounding the sinus is the largest, the others succes- 

 sively becoming smaller. Shell thick in the umbonal region, the 

 muscular scars being prominent on the internal casts. 



Brachial less gibbous than tlie pedicle valve, compressed to- 

 wards the cardinal extremities. Mesial fold highly elevated in 

 front, rounded or subangular, including four rounded plications 

 of which the two middle ones are generally, but not alwa3's, the 

 largest. In some cases two additional plications are present on 

 the fold as in the sinus. Sides of the valve ornamented with 

 plications as in the pedicle valve. Shell thinner than that of the 

 pedicle valve. 



This species is one of the most variable of Carboniferous 

 shells, the extreme variations as exhibited in the Batesville 

 specimens being shown in the accompanying figures. It has 

 sometimes been considered as identical with S. rockymontana, 

 Marcou,* but diflfers from the typical form as figured by Mar- 

 cou, in having coarser and fewer plications on the body of the 

 shell and on the fold and sinus, and in having a less angular 

 fold and sinus. S. rockymontana must undoubtedly be con- 

 sidered the successor of S. Jceokuk. All intermediate variations 

 occur in the higher beds of the Carboniferous, but these earlier 

 forms are constantly distinct from the typical S. rockymontana. 

 S. inc7^ebescens H. is another species which is also separated 

 with difficulty from S. keokuk, and may be merely a larger form 

 of the same species. 



Spiriferina sp. ? 



A single imperfect specimen belonging to this genus is present 

 in the collection. It is an internal cast of the pedicle valve, in 

 which the impressions of the median septum and dental lamellae 

 are well shown. The general form of the specimen as it is 

 preserved, is that of S. spinosa N. and P., but the characteristic 

 external markings of that species can not be seen in the Bates- 

 ville specimen. 



Athyris subquadrata Hall. 



PI. XIX., fig. 16. 



1858. Athyris subquadrata, Hall, Geol. Iowa, Yol. 1, pt. 2, p 

 103, PI. 2T,figs. 2a-d. 



*Geol. N. A., p. 50, PI. 7, flgs. 4-4e. 



