702 GEOLOGICAL SURVEY OF IOWA. 



BRACHIOPODA OF THE KASKASKIA LIMESTONE. 



AMONG the Brachiopoda of the Kaskaskia limestone, I have 

 selected a few of the most common and characteristic species, 

 for the purpose of carrying forward the means of comparison 

 in this class of fossils with those of the preceding formations. 

 The number of species here given might easily be doubled, 

 without exhausting the forms that seem to be everywhere 

 characteristic of the rock. 



So far as I have observed, the species of the Genus PRO- 

 DUCTUS are more rare, and of SPIRIFER perhaps more abundant ; 

 while those of ATHYRIS occur in greater number and in much 

 more numerous individuals, than in any of the lower rocks 

 of the Carboniferous period. 



Atliyris sublamellosa ( n . s .) . 



PLATE XXVII. FIG. 1 a, b, c. 



SHELL suborbicular, oval or ovate, more or less gibbous. 

 Dorsal valve regularly convex, gibbous in the middle ; beak 

 entering the foramen of the opposite valve : no mesial ele- 

 vation. Ventral valve convex in the middle above, depressed 

 below, often flattened in front, but without distinct sinus ; 

 beak extended beyond the opposite, scarcely incurved, trans- 

 versely or somewhat obliquely truncated by a round foramen. 



SURFACE marked by concentric lamellose imbricating striae, 

 which, when perfect, are extended in thin fimbriate expan- 

 sions. 



This species belongs to the group which includes Jlthyris roissyi, A. planosulcatus 

 and A. lamellosa ; differing from the former in the absence of a sinus, and from the 

 latter in the lesser convexity of the ventral valve, as well as other features. It is 

 possible that older specimens of this species may show a mesial sinus in front. 



