26 PALJEONTOLOGY OF NEW YORK. 



In Spirifer fastigatus, Meek and Worthen,* of the Keokuk group, the fascic- 

 ulntion of the plications is very pronounced, the duplication of these plications 

 beginning at an early stage in the development of the animal. In <S. Missouri- 

 ensis. Swallow, of the Choteau limestone, a similar fasciculation is apparent In 

 S. cameralus, Morton, of the Coal measures, this character becomes very conspic- 

 uous, while the extreme of development is attained in S. Texanus, Meek, where 

 the plications are sharply fasciculate, while the hinge is short, the fold and sinus 

 prominent and acute and the surface granulose. Spirifer fastigatus, S. cameratus 

 and S. Texanus might each well be taken as a type of structure of subordinate 

 value to that of S. slriatus, but in American faunas there are but these single 

 representatives of each variation.f 



(c) Imbrex-type. Alate, mucronate shells, with narrow cardinal area, fine, 

 simple ( very rarely duplicate ) lateral plications, the plications on fold and sinus 

 being of about the same size as the rest. The surface is frequently lamellose. 



This group is restricted to the earlier faunas of the Carboniferous and is 

 represented by 



S. Newberryi, Hall, Waverly group. 



S. Marionensis, Shumard, Waverly and Choteau groups. 



S. biplicatus, Hall, Kinderhook group. 



S. imbrex. Hall, Burlington limestone. 



S. incertus, Hall, Burlington limestone. 



S. Forbesi, Hall, Burlington limestone. 



S rostellatus, Hall, Keokuk group. 



S. tenuicostatus. Hall, Keokuk group. 



S. subaqualis. Hall, Keokuk and St. Louis groups. 



S. lateralis. Hall, Keokuk and St. Louis groups. 



(d) Suborbicularis-type. Forms with suborbicular outline, broad, low and 

 usually simple lateral plications ; the median plications are few and indistinct. 



S. suborbicularis. Hall, Kinderhook-Keokuk groups. 

 S. subcardiiformis, Hall, St. Louis group. 



• Aa this name waa preoccnpied by MoBToir, Mr. 8. A. Millbb has introduced in its place 8. Mortonanu». 

 t Waaobh has include<l in his "Group of Spirifer Ugitlatwi, Tiaulschold," S. Mnsakheylensis, Davidson, 

 •nd 8. Ambientit, Waa^en, all Carboniferous species of the Cameratus-type. 



