RHYNCHOTETRA 205 



plications low and obscure, originating in front of the umbonal region, 

 eight or ten may be detected in the specimen studied, all of which are 

 included within the broad median sinus, the two median plications are the 

 broadest, the others being much narrower. 



Brachial valve strongly convex transversely, the surface along the 

 median line describing a slightly sigmoidal curve, being a little convex 

 posteriorly and becoming slightly concave towards the front, from the 

 median line the surface curves steeply to either side, the least abrupt 

 curvature being towards the points of greatest lateral extension near the 

 mid-length of the valve ; mesial fold obscure posteriorly but in front of 

 the middle it includes nearly the entire width of the valve; beak small, 

 incurved beneath that of the opposite valve and nearly filling the delthy- 

 rium of that valve; plications obscure, similar to those of the opposite 

 valve. 



Surface of both valves marked by exceedingly fine concentric lines of 

 growth. 



Remarks. Only a single example of this species, from Bloomington, 

 Indiana, has come under the observation of the writer but this specimen is 

 apparently more perfect than either of those figured by Beede. At first 

 sight the shell seems to be entirely free from plications but a careful ex- 

 amination discloses them as they have been described. Material is not at 

 hand for the investigation of the internal features of the shell, but in all 

 of its external aspects it more closely resembles such typical members of 

 the genus Pugnax as P. acuminatus, than any other form which has been 

 referred to that genus from our Mississippian faunas. In most respects, 

 in fact, the shell is only an extremely minute example of P. acuminatus. 



Horizon. Salem limestone. 



Description. Shells usually of about medium size or larger, subovate 

 to triangularly subovate in outline, often subcuneate posteriorly, the 

 valves subequaliy convex or the brachial valve more convex than the ped- 

 icle, the mesial fold and sinus slightly developed or obsolete. Both valves 

 marked by broad, rounded or subangular plications which are either 

 simple or dividing, and when the surface is well preserved it is also 

 marked by fine, radiating striae ; upon the postero-lateral surfaces of the 

 shell the plications become obsolete or nearly so. Internally the dental 

 lamellae of the pedicle valve are strongly developed, becoming joined 

 near the floor of the valve to form a broad and deep spondylium which is 

 supported by a strong median septum and which continues anteriorly 

 towards the center of the valve. In the rostral portion of the brachial 

 valve a strong median septum is present which divides internally to form 

 the walls of a deep and rather narrow crural cavity, posteriorly the hinge- 



