194 MISSISSIPPIAN BRACHIOPODA 



lete in the posterior half of the valve, becoming rather broad and deep 

 anteriorly and somewhat produced in front in a linguiform extension 

 which curves strongly towards the opposite valve ; the beak .acutely 

 pointed, only a little incurved, produced posteriorly beyond the brachial 

 valve to a conspicuous degree ; delthyrium large and triangular, partially 

 closed by a pseudo-deltidium, with a large, subelliptical foramen; plica- 

 tions simple, obsolete towards the beak but originating in or near the 

 umbonal region, gradually increasing in strength until they become con- 

 spicuously subangular towards the margin, usually two or three are pres- 

 ent in the bottom of the sinus, more rarely one, four or five, upon each 

 lateral slope there are usually three but sometimes only two strong pli- 

 cations, with from one to four additional weaker ones towards the car- 

 dinal margin. Internally the teeth are supported by a pair of short dental 

 plates. 



Brachial valve more convex than the pedicle, the greatest convexity 

 near the middle, from the center of the valve the surface curves most 

 abruptly to the postero-lateral margins, more gently to the posterior 

 extremity and to the antero-lateral margins, and extends nearly horizon- 

 tally to the anterior margin; mesial fold obsolete posteriorly, becoming 

 somewhat strongly elevated in front; the beak incurved beneath that of 

 the opposite valve; plications entirely similar to those of the opposite 

 valve, three or four usually present on the fold- but sometimes two or 

 five, two or three strong ones on each lateral slope, and several weaker 

 ones. Internally the valve bears a median septum which is somewhat 

 rapidly reduced in elevation in passing from the beak anteriorly, becom- 

 ing entirely obsolete in about one-third or less the length of the valve, 

 posteriorly the septum is divided to form a crural cavity which does not 

 continue anteriorly beyond the hinge sockets, each side of the divided 

 septum is produced anteriorly into one of the lateral divisions of the hinge 

 plate, and these in turn are produced into the somewhat elongate crura 

 which curve towards the pedicle valve. 



Surface of both valves usually quite smooth aside from the plications, 

 except near the margin of adult shells where there may be several more 

 or less distinct lines of growth, sometimes a more or less distinct line of 

 growth is present in the umbonal region and when present it usually 

 marks the place of origin of the plications. 



Remarks. This shell is usually a prolific species wherever it is found. 

 It occurs most abundantly in the Pella beds of Iowa, but is also a con- 

 spicuous species in some of the Ste. Genevieve beds of Illinois. The 

 species may be easily recognized by its smooth shell, aside from the plica- 

 tions, by the conspicuously protuberant beak of the pedicle valve, and by 

 the inflected cardinal margin of the pedicle valve to form a false cardinal 

 area. 



