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 Jl. con- 

 gcsta, 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 Jl. 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 strise. 



Position and locality. In the lower part of the Clinton group at Lockport. 



(Collection of Col. Jewttt.) 



455. 26. ATRYP A BIDENS(n. «/>.). 



PL. XXIII. Fiff. 3 a, b, c, d. 

 Compare Terebratula hidentata, Dalman, Hisingek, &c. 

 Also Terebratula decemplicata, Murchison, Silurian System, pag-. 641, pi. 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 striffi ; beak of dorsal valve small, closely 

 incurved, and apparently perforate (from accident ■?). 



This species closely resembles the ./I. dentata* of the Trenton limestone, but differs in having 

 the mesial depression less profound and not so much elevated in front. Tliis 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 ditferences, 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 .1. duplicala of the Che- 

 mung group. 



