208 
PALAEONTOLOGY OF NEW-YORK. 
This species is associated with 8. varicosa ; but may be distinguished from that 
one by the more numerous and smoother plications ; a more depressed, smooth 
and rounded mesial sinus ; the anterior inclination of the area, and the propor¬ 
tionally greater length of the ventral valve, (which is a little greater than the dorsal 
valve in that species,) and much greater than the height of the area. 
It also bears some resemblance to 8. eurut&ines of Owen ; but in that one the 
area is vertical and considerably less in height than the length of the ventral valve, 
and the latter is longer than the dorsal valve ; and although a smaller shell, the 
8. segmenta has more numerous plications than 8. euruteines. Its nearest analogue 
is the 8. angusta of the Hamilton group. 
The figures 5, 6, 7 and 8, Plate 31, illustrate the form and proportions of this 
species : the cardinal extremities have been broken off. 
Geological formation and locality. In limestone of the age of the Upper Hel- 
derberg, at the Falls of the Ohio, and at Charleston landing, Indiana. 
Spirifera arctisegmenta. 
PLATE XXXI. 
Spirifer arctisegmentus : Hall, Tenth Report on the State Cabinet, p. 131. 1857. 
“ “ Description of New Pal. Fossils, p. 91. 1857. 
Shell transversely semioval; length less than one-third the width : 
hinge-line equal to the greatest width of the shell, and terminating in 
mucronate points. 
Ventral valve the more convex, most prominent at the umbo, from 
which it slopes regularly to the anterior and lateral margins : mesial 
sinus angular, and distinctly defined quite to the apex of the shell; 
beak not incurved. Area flat, a little inclined forwards, striated longi¬ 
tudinally ; fissure narrow and open to the apex. 
Dorsal valve depressed convex, scarcely flattened towards the cardinal 
extremities : the beak and central portion of the shell, together with 
the linear area, slightly incurved. 
The surface of the ventral valve is marked by eight or nine angular 
plications, which are slightly curved towards the front, and about three 
of them only reaching the apex; the remainder coalesce with an eleva¬ 
ted ridge which borders the area. The plications on the dorsal valve are 
pretty direct, the greater part of them terminating in the margin at a 
