490 MISSISSIPPIAN BRACHIOPODA 



of the mature shell, in the greater obsolescence of the mesial sinus of the 

 pedicle valve towards the beak, in the broader and flatter sinus ante- 

 riorly. 



The species was originally described from Chester, Illinois, but the orig- 

 inal type of the species, judging from the illustrations alone, was not an 

 entirely typical representative as shown by the many specimens from 

 Chester which have come under the observation of the writer. Hall 's 

 figure 2b represents a shell having the sinus originating much nearer the 

 beak than usnal, in this respect simulating G. trinuclea, the lateral slopes 

 of the valves, however, do not show the distinct trilobation of the shell 

 which is so common in that species. Examples of the Chester shell can 

 be selected which agree almost exactly with some specimens of C. trinuclea, 

 but the average characters of a large number of individuals of the two 

 forms are quite different. The species is perhaps just as closely allied 

 to C. argentea of the Pennsylvanian faunas, it is of about the same aver- 

 age size but usually has a somewhat flatter mesial sinus. 



Horizon. Chester group. 



COMPOSITA SULCATA n. sp. 



Plate LXXXII, Figs. 1-10 



Description. Shell below medium size, subquadrate in outline, wider 

 than long, the greatest width near the mid-length of the shell. The di- 

 mensions of a nearly complete specimen are: length of pedicle valve 20 

 mm., length of brachial valve 17.5 mm., greatest width 23.9 mm., thick- 

 ness 12 mm. 



Pedicle valve most convex in the umbonal region, the surface curving 

 abruptly and becoming a little inflected to the cardinal margin, gently 

 convex from the umbo to the antero-lateral margins ; beak rather short, 

 incurved; the mesial sinus originating at or near the beak, increasing 

 rapidly in depth towards the front w r here it becomes very profound and is 

 produced in a rounded anterior extension of the valve, it is illdefined lat- 

 erally and its width in front is sometimes equal to one-half the total 

 width of the valve, in the bottom it is rounded or subangular. 



Brachial valve about as deep or a little deeper than the pedicle, its 

 greatest depth near the middle, the surface curving somewhat abruptly 

 from the umbo to the cardinal margin, gently convex from the umbo to 

 the middle of the lateral margins and more strongly curved to the antero- 

 lateral margins ; along the median line the surface is convexly curved pos- 

 teriorly, becoming nearly straight posterior to the middle and so continu- 

 ing to the front; mesial fold little or not at all differentiated posteriorly, 

 becoming much elevated in front, not sharply defined laterally but sep- 

 arated from the lateral slopes by a distinct sinuosity of the shell on each 

 side. 



