TRENTON CONGLOMERATE OF RYSEDORPH HILL 21 



Observations. From P . sericeus this> form differs in the 

 shape of the valves and their internal characters, no transitions 

 being found between the two. In outline it approaches more the 

 P. transversalis of the upper Siluric, specially in the pro- 

 jection of the umbo beyond the cardinal line and in the rounded 

 cardinal ears, but it has quite different muscle scars in the 

 brachial valve. It also shows a certain similarity to L e p t e 1 1 a 

 decipiens Billings, from the Beekmantown horizon of Que- 

 bec; it differs from that form, however, in being more convex 

 and by its internal characters. Its nearest relation is undoubt- 

 edly with P. gibbosus Winchell and Schuchert, from the 

 middle Trenton of Minnesota, from which form it differs in its 

 surface characters, having 16 to 18 coarser striae to the six 

 or seven of the western form; in being still more gibbous 

 and abruptly so in the central part; and in its outline, in having 

 the umbo project beyond the cardinal line and possessing marked 

 subauriculate cardinal extensions. Judging from the drawings 

 the western form seems also to be less rounded and more sub- 

 triangular in outline. A small European form, which from the 

 description and figures furnished by Davidson x is apparently 

 related to our species, specially in the shape and relative size 

 of the muscle impressions is Leptaena s c i s s a Salter. It 

 occurs in the Oaradoc and Llandovery beds of Great Britain. 



christiania Hall & Clarke 

 Christiania trentonensis $p. nov. 

 PI. 2, fig. 2^6 

 The most interesting brachiopod of the fauna of the black 

 compact limestone pebbles is a new species of the rare genus 

 Christiania, thus far represented in North America only by a 

 Helderbergian form. 



Diagnosis. Shell small, convexo-concave, somewhat variable 

 in shape, rotundo-quadrate to rotundo-rectangular, sides sub- 

 parallel or slightly converging to the cardinal line; front rounded. 

 Hinge line straight, only slightly shorter than the greatest 



1 Foss. Bracb. Pal. soc. 1861. v. 24, pt 3, no. 4, p. 325. 



