484 MISSISSIPPIAN BRACHIOPODA 



mens from the matrix. The stronger development of the fold and sinus 

 towards the anterior margin of the shell, and the concavity of the pedicle 

 valve towards the lateral margins, are characters produced almost entirely 

 by the development of the concentric lamellose expansions from the 

 shell, when these are removed both of these characters largely disappear. 

 In the original description of the species the absence of a distinct mesial 

 sinus in the pedicle valve is mentioned, and this is true in specimens, 

 such as are illustrated by Hall, which have for the most part lost their 

 concentric lamellae. 



The shell described by McChesney as Athyris obvia, is said to differ from 

 C. sublamellosa in having the pedicle valve deeper than the brachial. The 

 type of McChesney 's species is lost, and in his later publications the 

 species is not figured and is not even mentioned, suggesting that it was 

 abandoned by the author. The species was described from the Chester 

 limestone, but late collections have shown that no such form is commonly 

 present in the fauna of that formation which can be considered as distinct 

 from C. suNamellosa, although individuals sometimes occur having the 

 pedicle valve most convex, a condition usually due to the distortion of 

 the shell. It therefore seems safe to consider McChesney 's species either 

 as unrecognizable or as a synonym of C. sublamcllosa. 



The two species described from the Chester formation by Swallow as 

 Spirigera clintonensis and S. americana are clearly synonyms of C. sub- 

 lamellosa, although the types of the species are apparently lost and cannot 

 be studied. The de&cription of S. americana exactly fits the usual form of 

 C. sublamellosa, and it is not infrequently possible to select examples having 

 a somewhat greater width than usual which correspond closely with the 

 description of 8. clintonesis. 



Horizon. Ste. Genevieve limestone and Chester group. 



Genus COMPOSITA Brown 



^Description. Shells small or of medium size, subovate, subquadrangu- 

 lar to subpentagonal in outline, the valves biconvex, with the fold and 

 sinus developed in the anterior portion of the shell or sometimes extend- 

 ing posteriorly to the umbonal region, both the fold and the sinus may 

 be marked by a rather sharp mesial sulcus. The surface of the valves 

 smooth or marked only by concentric lines of growth which are never 

 extended into lamellae. The beak of the pedicle valve incurved so as to 

 conceal the delthyrium, but the foramen is usually exposed, encroaching 

 upon the umbonal region of the valve. Internally the dental lamellae 

 and muscular sears resemble those of Athyris, but with the diductor 

 impressions usually more faintly developed. In the brachial valve the 

 hinge-plate is similar to that of Athyris, but with its posterior margin 



