SPIRIPERELLA 415 



20 to 25 depressed, rounded, simple plications which become successively 

 fainter towards the cardinal extremities, the last four or five often being 

 almost obsolete. Internally the dental lamellae are remarkably developed 

 as thick, bifoliate plates extending nearly to the front of the valve, be- 

 tween them, in the posterior portion of the valve, is a well developed 

 delthyrial plate rather deeply depressed below the surface of the cardinal 

 area with its free margin concave. 



Brachial valve not observed. 



Remarks. This species is a close ally of 8. plenus and might perhaps be 

 considered as only a variety of that species. The exceedingly strong, 

 bifoliate dental lamellae are alike in both forms, but the one here de- 

 scribed is proportionally much broader and shorter, with the pedicle 

 valve much less strongly curved in an antero-posterior direction, and with 

 a less strongly arched cardinal area. 



Horizon. Burlington limestone. 



SPIRIFERELLA NEGLECTA (Hall) 

 Plate LXIV, Figs. 5-9 ; Plate LXV, Figs. 1-4 



1858. Spirifer neglectus Hall, Geol. Iowa, vol. 1, pt. 2, p. 643, pi. 20, fig. 5, 

 1875. Spirifer neglectus Meek and Worthen, Geol. Surv. 111., vol. 6, 



p. 523, pi. 30, figs. Ic, 2a. 

 1884. Spirifera neglecta Walcott, Pal. Eureka Dist., p. 217, pi. 18, fig. 10. 



Description. Shell above medium size, usually wider than long, but 

 sometimes longer than wide, greatest width at about the mid-length of 

 the shell, the hinge-line short, the cardinal extremities rounded. The 

 dimensions of two individuals, the larger of which is somewhat distorted, 

 are: length 51 mm. and 42 mm., greatest width 60 mm. and 39 mm., 

 length of hinge-line 28 mm. and 21 mm., thickness 38 mm. and 28 mm., 

 height of cardinal area 11 mm. and 9 mm. 



Pedicle valve strongly convex or gibbous with its greatest convexity 

 posterior to the middle, sometimes well up towards the umbonal region, 

 the surface curving very abruptly from the umbonal region to the car- 

 dinal margin and less abruptly to the antero-lateral margins, the convex- 

 ity of the valve extending out to the cardinal extremities ; the beak pointed 

 and incurved; cardinal area narrow and high, arched, the curvature in- 

 creasing towards the beak, the inferior portion sloping posteriorly at an 

 angle of less than 110 degrees to the plane of the valve, divided by a large 

 delthyrium which occupies the central half of the total width of the 

 true area on the hinge-line, the actual surface of the area being limited 

 to two narrow bands which pass obliquely from the hinge-line to the beak, 

 the lateral margins of the area not defined, its surface passing laterally 

 without demarcation into a false cardinal area which rounds gradually 

 into the lateral slopes of the valve ; lateral slopes of the valve each bearing 



