90 [S 



ENATE 



the middle; the mesial elevation becoming defined below the middle of the valve, 

 and often margined by a defined groove which is distinctly marked on the front 

 of the shell. Surface marked by regular equal concentric laminae of growth, 

 which often, towards the front of the shell, become lamellose expansions. These 

 lamellae are often abruptly bent at the margins of the mesial fold and sinus. 

 The greatest length of the specimens examined is about six-tenths of an inch. 

 This shell bears some resemblance to small individuals of A. concentrica; but 

 is more distinctly trilobate, and never acquires the same dimensions. 



Geological formation and locality. In rocks of the age of the Hamilton group, 

 near Iowa city. My specimens were received from Rev. W. H.Bahris. 



This species, in the Hamilton group of the West, is a representative of the A~ 

 thyris spiriferoidcs of the same group in New-York, and is associated with Spirifer 

 submucronatus , S. inutilis, S. aspera, and others which have been noticed as 

 similar to our Hamilton species. The A thyris spiriferoides is a much larger shell, 

 with much less strongly developed mesial fold and sinus; while in all stages of 

 growth it has a more orbicular form, and the surface is less regularly laminated. 



SPIRIFER DUBIUS ( n.s.). 

 Shell ovoid, ventricose; cardinal extremities rounded t proportions varying from 

 length and breadth equal, to length greater than breadth; the thickness of both 

 valves about equal to two-thirds the width. Dorsal valve the less convex; mesial 

 fold defined towards the anterior margin, and marked by four or five plications 

 which coalesce and die out before reaching the apex : beak incurved. Ventral 

 valve more gibbous, becoming ventricose in old shells, highly arcuate above : 

 beak incurved, with summit extending above the opposite valve from one-fifth 

 to one-fourth the entire length of the shell, xirea short, with margins undefined : 

 foramen high. Surface marked by fourteen to twenty or more plications, which 

 show a tendency to bifurcate near the margin, and, in old shells, are entirely 

 obsolete on the upper part of the valves : plications crossed by imbricating 

 lamellae. 



This species, though having a foramen and other characteristics of Spirifer, 

 has nevertheless a median septum in the ventral valve; which, however, does not 

 join the dental lamellae, as in Ctstina. There is likewise a short median septum 

 in the dorsal valve. The longitudinal median septum of the ventral valve is not an 

 unusual feature, being shown in some Lower Helderberg species, particularly in 

 S. octocostatus (^Yol.iu, Pal. New-York). Jn some species referred to Cystina, 

 the dental plates join this septum, not at its summit, but below, leaving it pro- 

 jecting within the triangular cavity. 



Among these various modifications of Spirifer, we find, in some of the Devo- 

 nian and Carboniferous forms, a less distinctly fibrous structure, or a puncto- 

 striate texture, while the interior organization is apparently that of true Spirifer. 

 I am indebted for this species to the Rev. W. H. Barris of Burlington, Iowa. 

 Geological formation and locality. In the shaly limestones of the age of the 

 Hamilton group, near Iowa city, Iowa. 



STROPHODONTA PLICATA ( n.s.). 

 Shell semielliptical, length and width nearly equal, cardinal angles scarcely ex- 

 tending beyond the width of the shell. Ventral valve regalarly convex : beak 

 extending beyond the cardinal line; area sublinear, vertically striate, and the 

 inner njargin strongly crenulate; foramen closed. Dorsal valve concave, the 

 depression slightly less than the interior of the opposite valve. Surface marked 

 by strong rounded plications, which bifurcate on the dorsal valve, and are in- 



