SPIRIFER 313 



only slightly protuberant beyond the hinge-line. Surface marked by 

 plications similar in form and number to those of the opposite valve, 

 those of the mesial fold becoming much broader and less elevated 

 towards the front than those of the lateral slopes. 



A few rather faint lines of growth are present upon each valve, the 

 finer surface markings not preserved upon the type specimen. 



Remarks. This species is especially characterized by the extreme de- 

 velopment of the fold and sinus near the front of the shell, no other 

 species being present in our faunas which is at all comparable in this re- 

 spect. Another distinguishing character is to be found in the bifurcation 

 of the plications and their unusual width towards the front. 



Horizon. Burlington limestone. 



SPIRIFER SHEPARDI n. sp. 

 Plate XXXVIII, Figs. 6-8 



Description. Shell of medium size, subsemicircular in outline, the 

 greatest width along the hinge-line, the cardinal extremities angular, 

 sometimes mucronate. The dimensions of a nearly perfect pedicle valve 

 are : length 21 mm., width along hinge-line 40.5 mm., convexity 12.5 mm., 

 height of cardinal area 5 mm. The dimensions of a somewhat imperfect 

 brachial valve are : length 20 mm., approximate width 36 mm., con- 

 vexity 7 mm. 



Pedicle valve rather strongly convex, greatest convexity posterior to 

 the middle, the surface curving abruptly from the umbonal region to 

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

 usually a little compressed towards the cardinal extremities, sometimes 

 with an ill-defined sinuosity passing obliquely from each side of the beak 

 to the lateral margins just in front of the cardinal extremities ; beak 

 small, pointed, incurved; cardinal area of moderate height, arched, the 

 curvature increasing towards the beak, lateral margins sharply defined, 

 sloping from the beak to the cardinal extremities ; lateral slopes marked 

 by from 20 to 23 rounded plications, the first two or three on each side 

 of the fold and sinus may bifurcate, the others simple, towards 

 the cardinal extremities the plications become successively weaker, the 

 last two or three being sometimes very faint or obsolete ; mesial sinus of 

 moderate depth, rounded or subangular in the bottom, sharply defined 

 to the beak, somewhat loosing its definition anteriorly, but slightly pro- 

 duced in front, marked by. a median plication which originates near the 

 beak and passes to the anterior margin either without division or bifurcat- 

 ing, on each side there are from two to four plications which originate 

 from the inner margins of the lateral bounding plications or through the 

 bifurcation of plications so originating, making usually from 7 to 9 

 plications in all. 



