RHYNCHOPORA 229 



Brachial valve more convex than the pedicle, becoming gibbous in 

 mature shells, the umbonal region flattened longitudinally or sometimes 

 with a slight mesial depression, the greatest convexity at or near the front 

 margin, the surface sloping from the point of greatest convexity to the 

 beak with an increasing curvature and with a more or less abrupt curva- 

 ture to the front, laterally from the median line the surface is at first 

 gently convex but soon curves abruptly to the margins; the mesial fold 

 obsolete in the posterior half of the valve and only moderately or not at 

 all differentiated anteriorly ; the beak broad, incurved beneath that of the 

 opposite valve; the plications similar in form and number to those of the 

 opposite valve and alternate with them. Internally the cardinal process 

 is lacking and the hinge-plate is undivided, although the median portion 

 is differentiated from the lateral portions, it being thinner and usually 

 either a little concave or a little convex; a median septum is present 

 at the beak which is soon divided, one branch supporting each of the 

 lateral divisions of the hinge-plate, but because of the continuous hinge- 

 plate the pit between the divisions of the septum is not an open 

 crural cavity as in Camarotceckia, but is a covered excavation open only 

 towards the front ; a little posterior to the articulation of the hinge, the 

 median septum and its divisions which support the hinge-plate become 

 discontinuous, the septum continuing as a gradually disappearing ridge 

 for about one-fourth the length of the valve from the posterior extremity. 



Surface of both valves, apart from the plications, nearly or quite 

 smooth, except for a few obscure lines of growth near the anterior margin 

 of adult shells. The shell structure is minutely punctate and in at least 

 one specimen the perforations are arranged in longitudinal rows, a single 

 row upon each slope of each plication. 



Remarks. This species occurs abundantly in the Hamburg oolite 

 of Kinderhook age, at Hamburg, Calhoun County, Illinois. Externally, 

 it somewhat resembles, in size and general form, the Camarotcechia 

 chouteauensis from the Chouteau limestone, but its plications are less 

 angular, its fold and sinus are less abruptly differentiated from the 

 lateral slopes, and its beak is less incurved and more produced posteriorly. 

 Internally the two forms are fundamentally different, R. hamfiurgensis 

 having the continuous hinge-plate of the genus Rhynchopora, while in C. 

 chouteau&nsis the hinge-plate is medially divided and the characters are 

 those of Camarot&chia. The two species are also differentiated by the 

 punctate shell structure of R. hamburgensis, 



The internal characters of the species have been determined by the 

 study of serial sections made by grinding detached valves. These de- 

 tached valves have, of course, been subjected to accident during depo- 

 sition, and in no case have the crura been detected, although they were 

 undoubtedly present as anterior prolongations from the lateral divisions 

 of the hinge-plate. In some of the specimens examined the median por- 



