240 MISSISSIPPI AN BRACHIOPODA 



faint ; fine, regular, concentric lines of growth cover the entire surface of 

 the valve, becoming somewhat stronger towards the outer margin. In- 

 ternally the dental lamellae are short. 



Brachial valve much more strongly convex than the pedicle, the great- 

 est convexity near the anterior margin, the surface sloping with a long, 

 gently convex curve from the point of greatest elevation to the beak, a 

 portion of this slope in the middle part of the valve being nearly straight, 

 along the median line in the umbonal region towards the beak, the sur- 

 face is a little impressed in a slight median sinus, the slopes from the 

 median line to the lateral margins of the valve are convex and rather ab- 

 rupt, with a slight interruption in the slope anteriorly at the lateral bor- 

 ders of the mesial fold ; mesial fold originating near the middle of the 

 valve, not strongly differentiated from the general convexity except near 

 the anterior margin; the beak strongly incurved beneath that of the op- 

 posite valve ; the plications entirely similar in form and number to those 

 of the opposite valve, and alternate with them, and the concentric lines 

 of growth also similar. Internally a thin median septum is present in the 

 rostral portion of the valve which divides internally and is joined to the 

 two lateral divisions of the hinge-plate, the resulting crural cavity being 

 very broad and shallow. The hinge-plate apparently divided. Shell 

 structure minutely punctate. 



Remarks. The specimens originally used in the description of this 

 species were collected by Mr. D. K. Greger at Landing- No. 76, Perry 

 County, Missouri. Since the description was written, however, some ex- 

 cellent examples of the species have been collected from the Okaw lime- 

 stone of the Chester group in Randolph County, Illinois, which clearly 

 exhibit a distinctly punctate shell structure. A re-examination of the 

 Perry County examples has led to the detection of the same structure 

 upon these specimens, although it is very obscure and has been almost en- 

 tirely obliterated in the process of fossilization. The punctate shell struc- 

 ture in a rhynchonelloid shell is characteristic of Rhynchopora, in which 

 genus the species is here placed, although it differs in some particulars 

 from other species which have been included in this genus in this report. 

 In all the other species of Rhynchopora here recognized,, the hinge-plate 

 of the brachial valve is undivided, the crural cavity being inclosed above, 

 but in this species the hinge-plate is apparently divided. It is, of course, 

 possible that the median plate joining the two sides of the hinge-plate 

 was very delicate and easily injured, and that it has been destroyed in the 

 few specimens which have been ground down, but this is not likely, since 

 all of the specimens so examined have been shells with both valves in ar- 

 ticulation. So far as it is known, the internal characters of the species 

 are identical with those of Camarotoechia. Another character in which 

 this species differs from other members of the genus Rhynchopora here 

 recognized, is the obsolescence of the plications in the posterior portion 



