SPIRIFER 343 



moderately elevated in front and rather sharply differentiated from the 

 lateral slopes of the valve ; the beak short and a little incurved, the cardinal 

 area distinctly developed but much narrower than that of the opposite 

 valve, which it meets at an angle of about 90 degrees or a little less. 

 Surface marked by plications entirely similar in form and number to 

 those of the opposite valve, and by similar concentric markings. 



Remarks. This species has its nearest ally in 8. pellaensis, from which 

 it differs in its proportionately narrower and more elongate outline, and 

 in always having the hinge-line shorter than the greatest width. An 

 occasional specimen of S. pellaensis may approach very closely to this 

 species in outline, and so seem to unite the two forms by intermediate 

 variations, but the average form of S. pellaensis is much wider along the 

 hinge-line than the average form of this species, a fact which is believed 

 to be sufficient value to be considered as a specific difference. In the form 

 of the plications and other characters of surface ornamentation the two 

 species are essentially alike. 



Horizon. Chester group. 



SPIRIFER INCREBESCENS Hall 

 Plate XL VI, Figs. 1-12 



1858. Spirifer increbescens Hall, Geol. Iowa, vol. 1, pt, 2, p. 706, pi. 27, 



figs. 6a-i. 

 1858. Spirifer increbescens var. transversalis Hall, Geol. Iowa, vol. 1, pt. 2, 



p. 708. 

 1883. Spirifera increbescens Hall, Rep. N. Y. State Geol. for 1882, pi (30) 



55, figs. 27-30, pi. (31) 56, figs. 1-3. 

 1894. Spirifer increbescens Hall and Clarke, Int. to Study of Brach., 



pt. 2, pi. 27, figs. 8-11. 



1894. Spirifera increbescens Keyes, Mo. Geol. Surv., vol. 5, p. 82. 



1895. Spirifer increbescens Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, pi. 30, 



figs. 27-30, pi. 31, figs. 1-3. 

 1909. Spirifer increbescens Bassler, Va. Geol. Surv., 'Bull. No. II A, pi. 29, 



figs. 1-3. 

 1911. Spirifer increbescens Girty, Bull. U. S. Geol. Survey No. 439, p. 68. 



Description. Shell of medium size, wider than long, greatest width 

 along the hinge-line, the cardinal extremities angular, the valves sub- 

 equally convex. The dimensions of two individuals are: length 31 mm. 

 and 28 mm., greatest width along the hinge-line 40 mm. and 44 mm., 

 thickness 23 mm. and 19 nim. 



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

 surface sloping abruptly from the more or less gibbous umbonal region 

 to the cardinal margin and more gently to the antero-lateral margins, 

 usually somewhat compressed towards the cardinal extremities but 

 sometimes with the fullness extending throughout ; beak pointed and in- 

 curved; cardinal area concave, becoming more curved towards the beak, 



