456 MISSISSIPPIAN BRACHIOPODA 



terior margin is rounded ; the muscular scars are more or less ill-defined ; 

 a low, narrow, mesial septum similar to that of the pedicle valve origin- 

 ates a little in front of the base of the cardinal process and continues to 

 the middle or beyond the middle of the valve. 



Surface of both valves marked only by concentric lines of growth which 

 are commonly restricted to the anterior half of the shell and often occur 

 only close to the anterior margin. The delicate surface spines of the 

 genus have not been observed in any of the specimens studied, possibly 

 because of the condition of preservation. 



Remarks. This species is characterized by the development of a 

 stronger mesial fold and sinus near the front of the shell than is usual in 

 members of the genus. This character, with the not infrequent develop- 

 ment of rather strong concentric lines of growth towards the front, sug- 

 gests a smiall species of Athyris, but the internal characters, the cardinal 

 process of the brachial valve, and the low, elongate, median septum in each 

 valve show at once that its relationships are with the genus Nucleospira. 

 The species is also much the largest one which has been recognized in any 

 of our Mississippian faunas. The little shell from the Louisiana limestone 

 which has sometimes been identified as N. barrisi is an entirely distinct 

 species and is described in this report as N. rowleyi. 



Horizon. Upper Kinderhook. 



Plate LXXXII, Figs. 49-60 



1906. Nucleospira minima, Weller, Trans. St. Louis Acad. Sci., vol. 16, p. 

 447, pi. 6, figs. 29-30. 



Description. Shell very small, subcircular in outline, lenticular, the 

 two valves subequally convex. The dimensions of a large pedicle valve 

 are : length 5 mm., width 6.1 rum., convexity 1.6 mm. 



Pedicle valve with its greatest convexity posterior to the middle, the 

 surface curving more abruptly to the cardinal margin and gently to the 

 lateral and anterior margins, not compressed towards the cardinal ex- 

 tremities; the beak short and obtuse, incurved, projecting but slightly 

 beyond the cardinal margin ; mesial sinus originating at the beak, narrow 

 and groove-like ; internally a faint median septum is present in the rostral 

 portion of the shell and extends anteriorly to about the middle of the 

 valve. 



Brachial valve similar to the pedicle, its greatest depth posterior to the 

 middle, the surface not compressed towards the cardinal extremities; the 

 mesial portion of the valve narrowly flattened on the umbo and sometimes 

 slightly depressed as a narrow median sinus anteriorly. Internally a low 

 and narrow median septum continues from the beak nearly to the ante- 

 rior margin. 



