2 20 THE AMERICAN MIDLAND NATURALIST 



toward an abrupt mucronate development. The surface of the 

 shell is commonly marked by very heavy lines of growth. 



This form quite resembles in shape the form from the Ouray 

 illustrated and described in Bulletin 391 of the United States 

 Geological Survey. It attains a size considerably larger than the 

 typical Whitneyi of the Hackberry, and in fact any except those of 

 the west, so far as I have had opportunity to examine. The general 

 shape of the shell is similar to that oi Spirifer Whitneyi rockjordensis, 

 though the dorsal valve does not attain as great a development 

 compared with the ventral, nor does the sinus depart from its 

 regular form. The width of the shell in proportion to the thickness 

 is considerably less than in the typical Spirifer Whitneyi Hall. The 

 area is narrower and more curved, and the general appearance of the 

 shell is heavier than the true species. There is considerable vari- 

 ation, and it almost appeared that some of the specimens of this 

 form are transitional between Spirifer Whitneyi Hall and Spirifer 

 Whitneyi rockfordensis of this paper. It has been inadvisable to 

 select a holotype, due to variance in form. Cotypes, set 868, 

 collection of C. L. Fenton, Spirifer zone, pits of the Rockford 

 Brick and Tile Company, Rockford, Iowa. 



The illustrations will show in detail the distinctive features of 

 this form. It seems more prominent at Rockford than Hackberry 

 Grove, though at stations north and west of Rockford it is much 

 less common than at the brick pits. This is, I think, due to the 

 character of the exposure nearly as much as to irregular distribution 

 of the form. 



Spirifer Whitneyi productus nov. mutant. 



In general details of structure, number and arrangement of 

 plications and character of sinus this form would not differ from the 

 typical species in description. The number of plications is occasion- 

 ally larger on either side of the sinus and mesial fold, and they have 

 coarser character than in typical specimens from the Spirifer beds. 

 The important point of difference, however, rests in the fact that the 

 form assumes a very decidedly mucronate character, with a corres- 

 pondingly greater width as compared with the length of the shell, 

 and much greater length of area in proportion to its height. This 

 characteristic, while by no means lacking prominence is very 

 unstable in the extent of its development, and it is not uncommon 

 to find one tip produced far more than the other, and it is such a 



