246 MISSISSIPPIAN BRACHIOPODA 



Pedicle valve subcarinate along the median line, especially towards 

 the front, arched from beak to front with the curvature a little more 

 abrupt posteriorly, the lateral slopes gently convex from the median line 

 to the lateral margins; the margins of the valve between the cardinal 

 extremities and the apex of the beak inflected to form a slight pseudo- 

 cardinal area which lies in nearly the plane of the valve ; the beak prom- 

 inent, acutely pointed, very slightly incurved ; delthyrium rather broadly 

 triangular, partially closed by a pair of deltidial plates which do not 

 come in contact along the median line, thus leaving a triangular foramen 

 which reaches from the apex of the delthyrium to the base. 



Brachial valve less convex than the pedicle, with the greatest convexity 

 posterior to the middle ; an angular or subangular median sinus originates 

 in or just in front of the umbonal region and continues to the front margin 

 with increasing depth, the lateral margins of the sinus not defined; the 

 lateral slopes of the valve from the median line to the lateral margins 

 of the valve are transversely convex with the curvature a little more 

 abrupt towards the lateral margins, especially near the cardinal ex- 

 tremities; beak obtusely pointed, scarcely or not at all incurved. 



Surface of both valves marked by from eight to twelve rather broad, 

 rounded, simple plications which originate in the umbonal region and in- 

 crease in strength anteriorly, and by several more or less indistinct lines 

 of growth which are sometimes nearly or quite obsolete. 



Remarks. The internal characters of neither valve of this species have 

 been observed, so that the form of brachidium, if such a structure be 

 present, is not known. Neither has the punctate shell structure been 

 certainly determined. In its external features the general form of the 

 shell is notably centronelloid, and being plicated it is placed in the genus 

 Trigeria where it was first located by Rowley, the author of the species. 

 The specimen whose dimensions have been given above is perhaps of 

 average size, the larger examples attain a length of 4 mm. or more. 



Horizon. Louisiana limestone. 



Family TEREBRATULID^E 

 Genus CENTRONELLOIDEA n. gen. 



Description. Shell small, centronelliform, with smooth valves, subovate 

 to subcuneate in outline. Pedicle valve with a longitudinal median ridge 

 from which the lateral surfaces slope rather abruptly to the margins, 

 the beak acuminate and moderately incurved, with a circular foramen 

 at the apex, the delthyrium closed by the pseudodeltidium. Internally 

 the dental lamellae are well developed but are of only moderate length. 

 The brachial valve much shallower than the pedicle with a longitudinal 

 median sinus. Internally the hinge plate is divided medially, the two 

 lateral divisions being supported by a pair of septa which rise from the 



