68 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



costae beginning about 7 mm. anterior to beak, along with fold and 

 sulcus ; concentric growth lines fine, growth laminae weak, only slightly 

 more prominent than growth lines ; edges of valves meeting in acute 

 angle in flat juveniles, obtuse angle in convex adults, not protruding, 

 without forming trace of stolidium. 



Pedicle valve strongly convex (in adults) beak thick, rather long, 

 erect to strongly incurved, pressed against brachial umbo of large 

 adults ; beak ridges short, rounded ; foramen small, constricted by 

 curvature of beak, apparently completely closed in large adults. 



Brachial valve strongly convex, beak somewhat attenuate, apex 

 within pedicle valve. 



Pedicle valve interior with dental plates uniting above floor to form 

 shallow spondylium, elevated on low median septum duplex in apex, 

 septum becoming rather high anteriorly, leaving spondylium shallow, 

 extending anteriorly about one-third length of valve. 



Brachial valve interior with undivided hinge p!ate, swollen in apex 

 to form cardinal process with shallow longitudinal striae for muscle 

 attachment; camarophorium with thick trough, supported on thick 

 median septum duplex extending anteriorly about one-fourth length of 

 valve ; intercamarophorial plate bracing camarophorium to hinge plate. 



Holotype.— USNM 142071, plate 8, fig. 3a. 



Comparisons. — Camerisma girtyi is characterized by its large size, 

 bulbous convexity, high fold with bluntly pointed crest, strongly 

 curved pedicle beak, and weak costae, one on each side of the fold. It 

 is larger and narrower than the other known Mississippian species 

 from Alaska, C. prava n. sp., and differs further in its greater convexity 



A B 



Fig. 12. — Camerisma girtyi n. sp., Mississippian, Ladrones Ids., Alaska, 

 transverse sections X4, USNM 142070, original length about 19.0 mm. 



A. About 3 mm. from pedicle valve beak : cardinal process low and narrow, in- 

 tercamarophorial plate independent of camarophorium, septa both duplex, with 

 wedged insertions into valve floors, ventral septum and lower part of spondylium 

 secondarily thickened. B. Slightly farther anterior, dental plates lower. 



