BRACHYTHYRIS 371 



the hinge-teeth are short, commonly being only ridge-like thickenings of 

 the inner surface of the valve on each side of the delthyrium, the muscu- 

 lar scars ill-defined. The brachial valve commonly less convex than the 

 pedicle, with a very narrow, inconspicuous cardinal area, the cardinal 

 process low, with its anterior surface vertically striated : the form of the 

 brachidiumi as in Spirifer. 



Remarks. The genus Brachythyris was proposed by McCoy in 1862, for 

 the ovate, short-hinged Spirifers of which 8. ovalis Phill. was figured as a 

 typical form, and which must be assumed as the genotype, as has been 

 pointed out by Buckman. 1 The shells of this type constitute a well de- 

 fined group of species which may be conveniently separated from Spirifer 

 proper, although it has usually been the custom to consider the name as 

 strictly synonymous with Spirifer. In the American Mississippian faunas 

 Spirifer siiborbicularis is a typical member of the genus. 



BRACHYTHYRIS BURLINGTONENSIS n. sp. 

 Plate LIII, Figs. 8-9 ; Plate LXXXIII, Fig. 6 



1901. Spirifer peculiaris 1 Weller, Trans. St. Louis Acad. Sci., vol. 11, 

 p. 165, pi. 14, figs. 6-9. 



Descript ion. Shell small, longitudinally semielliptical in outline, the 

 hinge-line usually a little shorter than the greatest width of the shell, the 

 greatest width posterior to the middle, the cardinal extremities usually 

 rounded. The dimensions of a small, nearly complete internal cast, are : 

 length of pedicle valve 11.5 mm., length of brachial valve 11 mm., great- 

 est width 13.3 mm., length of hinge-line 10 mm., thickness 9 mm., height 

 of cardinal area 2 mm. The dimensions of a larger pedicle valve are: 

 length 19.5 mm., greatest width J8 mm., length of hinge-line 13 mm., con- 

 vexity 8 mm., height of cardinal area 2.5 mm. 



Pedicle valve strongly convex, the greatest convexity posterior to the 

 middle, the umbo prominent, the surface curving abruptly to the cardinal 

 margin and more gently to the antero-lateral margins, slightly or not at 

 all compressed towards the cardinal extremities; beak small, pointed, in- 

 curved; cardinal area small, arched, the curvature increasing towards 

 the beak, lateral margins sloping from the beak to the cardinal extremi- 

 ties, scarcely defined, the surface of the area curving with little or no 

 demarcation into the lateral slopes of the valve; lateral slopes convex, 

 marked by from 8 to 10 simple, rounded, depressed plications which grow 

 fainter towards the cardinal extremities; mesial sinus narrow, shallow, 

 flattened in the bottom, sometimes but little more than a flattening of 

 the valve along its median line, bounded by a pair of plications which are 

 notably broader and stronger than those of the lateral slopes, smooth in 



i Science, new ser., vol. 26, p. 379. (1907.) 



