RRACHYTHYRIS 381 



Remarks. This species was originally described in a very inadequate 

 manner without illustrations, from the Rockford limestone of Indiana, 

 and the type seems to have been lost. The little shell which is illustrated 

 herewith, and which has been used as a basis for the above definition, is 

 from a limestone in Union County, Illinois, which is believed to be an ex- 

 tension of the Rockford limestone of Indiana: It seems to agree in all 

 essential respects with Hall's original description of the species, and has 

 consequently been so identified. 



Horizon. Rockford limestone of the Kinderhook. 



BRACHYTHYRIS PECULIARIS (Shumard) 

 Plate LVII, Fig. 12 ; Plate LVIII, Figs. 9-20 ; Plate LXXXIII, Figs. 3-5 



1855. Spirifer ? peculia.ris Shumard, Geol. Rep. Mo., 1855, p. 202, pi. C, 



figs. 7a-b. 

 1894. Spin/era peculiaris Keyes, Mo. Geol. Surv., vol. 5, p. 79. 



Description. Shell subrhomboidal in outline, usually wider than long, 

 the greatest width at about the mid-length of the shell, the hinge-line 

 about one-half as long as the greatest width, cardinal extremities rounded, 

 the beaks of the two valves remote. The dimensions of two perfect indi- 

 viduals, one with average proportions, the other much more elongate than 

 usual, are : length of pedicle valve 16.5 mm. and 18 mm., length of brachial 

 valve 15 mm. and 16.5 mm., greatest width 19 mm. and 16 mm., length of 

 hinge-line 9.5 mm. and 8 mm., thickness 14 mm. and 15 mm., height of 

 cardinal area 4 mm. and 3.2 mm. 



Pedicle valve strongly convex, the umbo prominent, the surface curving 

 very abruptly to the cardinal margin and more gently to the antero- 

 lateral margins ; the beak acuminately pointed, rather small, strongly in- 

 curved ; cardinal area rather high and narrow, the inferior portion direct- 

 ed in nearly a right angle to the plane of the valve, the upper portion 

 strongly arched, the lateral margins sloping steeply from the beak to the 

 extremities of the hinge-line, scarcely defined from the inflected parts 

 of the lateral slopes which form a false cardinal area ; each lateral slope 

 divided into two regions by a rounded ridge which passes from the beak 

 to the point of greatest lateral extension of the valve, on the posterior 

 side of this ridge the surface is abruptly inflected and forms one side of 

 a false cardinal area which is scarcely differentiated from the true car- 

 dinal area, the anterior slope on each side is marked by from 5 to 8 simple, 

 depressed, rounded plications which terminate posteriorly along the 

 rounded ridge between the anterior and posterior portions of the lateral 

 slope ; mesial sinus narrow, shallow, rounded in the bottom, continuing to 

 the beak, without plications. 



Brachial valve much less convex than the pedicle, the umbo prominent, 

 the beak projecting conspicuously beyond the hinge-line posteriorly; the 



