CLINTON GROUP. 
69 
have become prominent; but in that species these folds never reach the base of the shell, while 
they do so even in young individuals of the present species. In older individuals of A. con¬ 
gesta, the front of the ventral valve is more elevated and the sinus deeper than in this shell ; 
while in the single young individual of this species known, the sinus is developed more than 
in the young of A. congesta. The cardinal line in A. quadricostata is much more extended than 
in A. congesta , and has more the appearance of a Spirifer. The species are therefore un¬ 
questionably quite distinct, as will be seen on comparing individuals. 
Fig. 2 a , b. Ventral and front view of a young shell of this species. 
Fig. 2 c, d. Ventral and front view of an older individual. 
Fig. 2 e. Profile view of the same. 
Fig. 2f. A portion of the surface enlarged, showing the striae. 
Position and locality . In the lower part of the Clinton group at Lockport. 
(Collection of Col. Jewett.) 
455. 26. ATRYPA BIDENS (n. sp .). 
Pl. XXIII. Fig. 3 a, b, c, d. 
Compare Te.re.bratula bidentata, Dalman, Hisinger, &c. 
Also Terebratula decemplicata, Murchison, Silurian System, pag. 641, pl. xxi, fig. 17. 
Subpyramidal, oblate, the width greater than the height; valves subequal, the dorsal valve 
slightly more convex and extended at and near the beak ; cardinal line not extended ; margins 
abruptly sloping from the beak downwards ; dorsal valve with a profound mesial depression, 
in the centre of which is a single small plication ; front much elevated, and filling a sinus in 
the ventral valve ; ventral valve with two plications on the mesial elevation ; each valve with 
about five angular plications on each side of the mesial fold (rarely six on the dorsal valve) ; 
surface marked by fine concentric filiform undulating striae ; beak of dorsal valve small, closely 
incurved, and apparently perforate (from accident?). 
This species closely resembles the A. dentata* of the Trenton limestone, but differs in having 
the mesial depression less profound and not so much elevated in front. This species has like¬ 
wise five or more plications on each side the mesial fold ; while in that one there are but four 
on each side, though the specimen described is larger than those here figured. In the Tren¬ 
ton species the sinus is wider at the base, and the extension of the dorsal valve higher in front. 
Notwithstanding these differences, as well as a general difference in the aspect of the shells, 
they may readily be confounded on cursory examination. 
*This species has been regarded by several palaeontologists as identical with A. bidentata of Dalman. I have not 
been able to discover the identity of the two, on comparing our species with Swedish specimens. The resemblance is 
the same as between those and the one now described, or between either of them and the A. duplieata of the Che¬ 
mung group. 
