106 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



shows as a dark line on the surface and the spondylium is often ex- 

 posed by the breaking away of the beak. 



Brachial valve more convex than the pedicle, the greatest depth 

 anterior to the middle. Slope from the middle to the beak and margins 

 about equal except on the mesial fold ; the mesial fold begins about the 

 middle of the valve and has a single ill-defined furrow. Beak pointed 

 and incurved beneath that of the pedicle valve. 



Ordinarily the lateral slopes are without plications, but in a few of 

 the largest specimens a very faint plication is developed at the an- 



Fig. 23. — Coledium cestriensis (Snider), Pitkin Limestone, Okla. (Waller loc. 

 169-H, unit 34), USNM 142105. Longitudinal section through midline, X5, 

 showing profiles of camarophorium, intercamarophorial plate (lined), hinge plate, 

 and septum of spondylium. 



terior margin near the mesial fold on the brachial valve and a corre- 

 sponding furrow is shown on the ventral valve. 



Surface markings consist of faint concentric lines of growth. 



Supplementary descriptive remarks. — Spondylium in pedicle valve 

 sessile at extreme apex, then elevated on low median septum ; trough 

 of spondylium extending anteriorly about one-third length of valve, 

 septum continuing nearly to midlength. Camarophorium in brachial 

 valve extending to midlength of shell, about one-third length of valve, 

 septum supporting camarophorium slightly shorter ; trough strongly 

 curved ventrally (longitudinally), producing very high septum be- 

 tween it and strongly curved valve floor ; hinge plate short, connected 

 to posterior part of camarophorium by thin intercamarophorial plate. 



Lectotype {here designated). — Snider, 1915, pi. 4, figs. 15-18 

 (Univ. Chicago coll.). 



Growth. — The patterns of growth in this species are typical for the 

 genus, with increase in width varying narrowly along a straight line 

 relative to increase in length, and increase in thickness slightly ac- 

 celerated (fig. 24). The slope of the length- width line shows that short 



Fig. 24. — Scatter diagrams of dimensions of Coledium cestriensis from all listed 

 localities in the Pitkin Formation, Okla. 



