200 MISSISSIPPIAN BRACHIOPODA 



plications simple, rounded, 14 to 22 in number, two or three of which on 

 each side adjacent to the postero-cardinal margins are very faint, they 

 are obsolete at the beak but originate in the umbonal region, usually at 

 the first strong concentric line of growth. Internally the hinge-teeth are 

 supported by a pair of short dental lamella which do not extend anteriorly 

 beyond the articulation of the hinge. 



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FIG. 12. A series of four cross-sections of the rostral portion of the shell of 

 Allorhynchus macro, (X 2%), showing the dental lamellae of the pedicle valve 

 and the ahsence of lamellae in the brachial valve. 



Brachial valve equally or a little less convex than the pedicle, the 

 greatest convexity posterior to the middle, anteriorly the surface curves 

 gently to the anterior and antero-lateral margins ; posteriorly the surface 

 first curves gently from the median line laterally and then rather abruptly 

 as it approaches the postero-lateral margins; mesial fold obsolete; in 

 younger specimens and in the older part of mature ones there is a distinct 

 but slight depression along the median line which continues to the front 

 of full grown examples as a broad mesial flattening which is not sharply 

 differentiated from the lateral slopes of the valve ; the beak is rather 

 sharply incurved beneath that of the opposite valve; the plications are 

 entirely similar in form and number to those of the opposite valve. In- 

 ternally the hinge-plate is divided to the base and no median septum is 

 present, anteriorly the two divisions of the hinge-plate are produced 

 as crura. 



The surface of both valves, in addition to the plications, is marked by 

 exceedingly fine concentric striae and by several stronger lines of growth 

 which are sometimes placed at nearly regular intervals throughout the 

 entire length of the shell, the plications commonly originate at the first 

 of the lines of growth in front of the beak. 



Remarks. This species may be easily recognized by its small size and 

 much compressed shell. The smaller examples may sometimes resemble 

 the, young of the associated Camarotcechia mutata, but that species is 

 apt to have a more pronounced sinus in the anterior margin of the pedicle 

 valve, and the internal characters of the two species are different, the 

 median septum of the brachial valve with its crural cavity sup- 

 porting the bases of the divided hinge-plate being absent in the present 

 species. Rhynchonella ricinula Hall, seems to be only the young of the 

 larger R. macra, and is so considered here, although Whitfield states that 

 mature examples of R. macra do not occur with R. ricinula. In the col- 

 lections from the Salem limestone above Alton, Illinois, the typical locality 

 for R. macra, small examples which seem to agree fully with the figures 



