LA MELLIBRANCHIA TA. 
395 
SpIIENOTUS ARCiEFORMIS. 
PLATE LXV, FIGS. 7-11 ; AND PLATE LXVI, FIG. 13? 
Sanguinolites arcdfonnis. Hall. Prelim. Notice Lamellibranchiata, 2, p. 40. 1870. 
“ “ “ Pal. N. Y., vol. v, pt. 1. Plates and Explanations : PI. 65, fig's. 7-11. 1883. 
“ JEolus , Hall (in error). Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 66, fig. 43. 
1883. 
Shell of medium size, trapezoidal; length usually more than twice the height; 
basal margin gently curved, sometimes nearly straight in the middle. Pos¬ 
terior extremity obliquely truncate. Cardinal line straight, slightly declin¬ 
ing posteriorly. Anterior end short, sloping rapidly from the beak and 
narrowly rounded below. 
Valves becoming gibbous in the middle and umbonal region. 
Beaks at about the anterior fourth, small and closely appressed. Umbonal 
slope distinctly angular, extending to the post-inferior extremity. Post¬ 
cardinal slope concave, and marked by a scarcely defined fold which is nearer 
to the umbonal ridge than to the cardinal margin. Most of the specimens 
show a flattening or undefined depression extending from the beaks to the 
basal margin. 
Surface marked by extremely fine concentric strim, which sometimes 
become fasciculate in the anterior portion of the shell, and are undulating 
on the post-cardinal slope; also by fine radiating strife, which are rarely 
well-preserved, and are mainly confined to the posterior half of the shell. 
Five specimens measure respectively 23, 28, 32, 26 and 26 mm. in length, 
and 11, 13, 14, 14 and 11.5 mm. in height. 
This species differs from S. cuneatus in its smaller size, narrower posterior 
extremity, more convex basal margin, less conspicuous post-cardinal ridge, and 
in the surface characters. 
Formations and localities. In the shales of the Hamilton group, near Delphi, 
at Pratt’s falls and on the shore of Skaneateles lake, Onondaga county, N. Y.; 
and doubtfully in the Chemung group near Smethport, Pa. 
