ATHYRIS OF THE HAMILTON GROUP. 985 



primary lamellae 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 ia a conical 

 form. 



Atlijris spii'iferoides. 



PLATE XLVL 



Terebratula tpiriferoides : Eaton, American Journal of Science, Vol. xxi, p. 137."1831. 



" " Id. Geol. Text Book, p. 46. 1832. 



jtrr/pa concentrica i Conbad, Annual Report on Pal. New- York, pp. Ill, 112. 1838. 



" " Hall, Report on Fourth Geol. District New- York, p. 198, f. 5. 1843. 



Spirigera spiri/eroides : (Eaton, sp.). Hall in Tenth Report on the State Cabinet, p. 153. 1857. 

 I Terebratula concentrica : Vos BucH, Ueber Terebratula, T. 103, Berlin, 1884. Mem. Soc. Geol. France, 



Vol. Hi, p. 214. 1839. 

 Spirigera concentrica : Billings, Canadian Jour. 1801, 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 g-' .- 



• 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 lamellae 

 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 be much against their preservation entire. 



