708 GEOLOGICAL SURVEY OF IOWA. 



Spirifer increbcscens, var. transversalis. 



SHELL somewhat semicircular, twice as wide as long, gib- 

 bous in the middle, attenuate at the extremities, which are 

 prolonged into mucronate points. Dorsal valve marked by 

 a prominent mesial fold of four rounded plications, which 

 proceed from the bifurcation of a single one at the beak. 

 Ventral valve with a well-defined sinus, which is marked by 

 three plications of which the central one is the strongest ; 

 beak small, and neatly incurved over the apex of the broad 

 foramen ; area of medium height, slightly concave and 

 strongly striated vertically, extending to the cardinal extre- 

 mities : foramen wider at base than the length of the side. 



SURFACE marked by distant imbricating lines of growth 

 and finer concentric strise, which are crossed by equally 

 fine radiating strise. 



This shell agrees with the preceding in all its essential characteristics, except in 

 being narrower and more extended on the hinge-line. This form is always smaller; 

 and I can only regard it as an intermediate stage of growth, in which it differs no more 

 from the larger forms than does Spirifer mucronatus in its various stages of growth- 

 It has sometimes been referred to <S. strnngwaysi of MTJRCHISON and DE VERNEUIL, 

 but differs from that species in having the plications on the mesial fold and sinus 

 smaller instead of larger than those on the sides of the shell, and the lateral plications 

 never bifurcate near the border as in that species. 



The figures 6 a, b of PI. xxvii may be considered as representatives of this type, 

 which is often narrower and proportionally more extended. It is always associated 

 with the larger forms, or occurs in the same beds. 



FIG. 118. 



Athyris subquadrata. Front view, showing the elevation of the mesial fold and the 

 deep sinus in front. This feature varies with age, from a very moderate or scarcely 

 perceptible elevation and sinus, to the extreme character here shown. 



