NUCLEOSPIRA 455 



NUCLEOSPIRA BARRISI White 

 Plate LXXXII, Figs. 27-38 



1860. Niideospira barrisi "White, Jour. Boston Soc. Nat. Hist., vol. 7, p. 



227. 

 1901. Niideospira barrisi Weller, Trans. St. Louis Acad. Sci., vol. 11, p. 



199, pi. 20, figs. 7-11. 



Description. Shell small, wider than long, the greatest width near the 

 mid-length, transversely subelliptical in outline, the hinge-line very 

 short, the cardinal extremities rounded. The dimensions of a complete 

 pedicle valve, one of the co-types, are : length 9 mm., greatest width 10.6 

 mm., convexity 2.2 mm. The dimensions of a brachial valve, also one 

 of the co-types, are : length 8.7 mm., greatest width 11.2 mm., convexity 

 3.1 mm. 



Pedicle valve moderately convex, the greatest convexity near or poste- 

 rior to the middle, the surface sloping with a gently convex curvature to 

 the lateral and anterior margins and curving somewhat more abruptly 

 to the cardinal margin on either side of the beak; mesial sinus originat- 

 ing in the posterior half of the valve, sometimes nearly at the beak, nar- 

 row and shallow but often becoming deeper and broader near the front 

 margin and a little produced in a short, rounded extension; beak small, 

 only a little produced beyond the cardinal margin and only moderately 

 incurved; cardinal area wanting, the delthyrium rather large, nearly 

 filled by the beak of the opposite valve, the pseudodeltidiumi inconspicu- 

 ous, deeply concave and limited to the apical portion of the delthyrium. 

 Internally the diductor muscular scars are large and flabellate, but are 

 scarcely differentiated at their margins, the adductor scars are much 

 smaller and in mature shells form a narrow, elongate depression in the 

 inner surface of the valve; a low, narrow, mesial septum originates near 

 the apical portion of the valve, in the midst of the adductor scar, and 

 continues nearly to the front margin. 



Brachial valve subequally or a little more convex than the pedicle, the 

 greatest convexity near the middle, the surface curving more abruptly to 

 the cardinal margin and gently to the lateral and antero-lateral margins ; 

 the mesial fold originating near the middle of the valve, appearing 

 broader than the sinus of the pedicle valve, somewhat abruptly elevated 

 in front in mature shells to accommodate the anterior mesial extension 

 of the pedicle valve, sometimes with a very slight, line-like, mesial depres- 

 sion for a short distance back from the anterior margin in full grown 

 examples. Internally the cardinal process is prominent, and is curved over 

 the margin of the valve and projects posteriorly beyond the margin, 

 nearly filling the umbonal cavity of the pedicle valve w r hen the two valves 

 are in articulation, the surface of the process directed towards the um- 

 bonal region of the opposite valve bears a pit-like excavation and its pos- 



