ATHYRIS OF THE HAMILTON GROUP. ’ 
@85 
primary lamellce of the spire in its first volution.* The lamellae are 
represented as cut away at i, showing but a single volution. The remain¬ 
ing portions of the spires consist of simple volutions arranged in a conical 
form. 
Atliyris spiriferoides. 
PLATE XLYI. 
Terebratula spiriferoides : Eaton, American Journal of Science, Vol. xxi-, p. 137."1831. 
“ “ Id. Geol. Text Book, p. 46. 1832. 
Mrypa concentrica : Conrad, Annual Report on Pal. New-York, pp. Ill, 112. 1838. 
“ “ Hall, Report on Fourth Geol. District New-York, p. 198, f. 5. 1843. 
Spirigera spiriferoides : (Eaton, sp.), Hall in Tenth Report on the State Cabinet, p. 153. 1857. 
? Terebratula concentrica : Von Buch, Ueber Terebfhtula, T. 103, Berlin, 1834. Mem. Soc. Geol. France, 
Vol. iii, p. 214. 1839. 
Spirigera concentrica : Billings, Canadian Jour. 1861, and Geology of Canada. 1863. 
Shell varying from transversely oval to suborbicular and sometimes 
subquadrate, depressed or subglobose, more or less deeply sinuate on 
the ventral side, with a corresponding elevation on the dorsal side : 
hinge-line short; the cardinal extremities rounded. 
Ventral valve moderately gibbous, often regularly convex above the 
middle, and becoming deeply sinuate towards the front, which is fre¬ 
quently abruptly elevated, flattened or a little concave towards the 
cardinal extremities in the wider specimens : umbo gibbous, the beak 
incurved, and often directed in a line nearly rectangular to the.plane 
of the longitudinal axis, and covering the umbo of the opposite valve; 
apex perforate. 
Dorsal valve gibbous, much more convex than the opposite valve; umbo 
prominent, outline regularly convex above the middle and curving 
* Although expressing no doubt regarding the investigations of European species which have shown 
the free termination of the accessary lamellae, it seems to me more natural and as serving a practical 
purpose in the economy of the animal, to have them attached to the primary lamellae of the spires for 
the purposes of support and strength. It should not be forgotten, however, that the accessary IStmellas 
are very slender near their point of junction, and may be easily separated or broken, and it requires 
the greatest caution to preserve them entire. In specimens where these appendages are exposed from 
natural causes, the chances would he much against their preservation entire. 
