476 MISSJSSIPPIAN BRACHIOPODA 



Brachial valve somewhat more convex than the pedicle, the greatest 

 convexity towards the anterior margin, the surface strongly convex on 

 each side of the fold and sinus, the curvature being most abrupt to the 

 cardinal margin and least to the lateral margins ; mesial fold obscure or 

 obsolete towards the beak, becoming strongly elevated anteriorly, rounded 

 on top and not sharply denned laterally ; beak strongly incurved and 

 filling the delthyrium of the opposite valve. 



Surface of both valves, as usually preserved and when not greatly 

 exfoliated, marked by crowded, more or less regular, thin, imbricating 

 lamellose lines of growth, which are often somewhat wavy along their 

 free margins. In rare instances the concentric lamellae are seen to be 

 produced into thin, rather broad, lamellse, which are divided by grooves 

 or slits into ribs or spines, giving to the shell the essential characteristics 

 of the genus CliotJiyridina. 



Remarks. This species has commonly been confused with C. incrassata. 

 The two forms are closely allied, but they seem to be sufficiently distinct 

 to warrant their recognition as separate species, although it is probable 

 that some intermediate examples may be found. C. obmaxima- grows 

 to a larger size than C. incrassata, sometimes attaining a width of 75 mm. 

 or more; it is always broader than long, with a profound sinus which 

 originates nearer the beak than that of C. incrassata. The species occurs 

 most commonly in the Fern Glen formation and in the Keokuk, while 

 C. incrassata has been observed only in the Burlington limestone. The 

 shell described as Spirigera pectinifera by Swallow, has never been illus- 

 trated, and the types are lost, but the description is a good one and closely 

 applies to an example of C. obmaxima, upon which some of the delicate 

 surface markings are preserved, the species is recorded from Keokuk, Iowa, 

 from which locality many excellent examples of C. obmaxima have been 

 secured. 



Among European species the shell described by Phillips as Spirifera 

 glabristria, 1 is perhaps identical with our American C. obmaxima. David- 

 son has included the form in his broad interpretation of Athyris roissyi, 2 

 and DeKoninck has described a new species under the name Athyris ing ens? 

 which is a close ally. Phillips' specific name has priority, and if the 

 American shell should be considered as identical with the European, the name 

 C. glabristria Phill., would have to take precedence over C. obmaxima 

 McChesney. 



Horizon. Upper Kinderhook, Burlington limestone and Keokuk lime- 

 stone. 



iGeol. Yorks., vol. 2, p. 220, pi. 10, fig. 19 (1836). 



2 Brit. Foss. Brach., vol. 2, p. 84, pi. 18, figs. 1-4 (1860). 



3 Faun, du Calc. Carb. de la Belg., pt. 6, p. 85, pi. 20, figs. 1-10. (1887.) 



