288 
PALAEONTOLOGY OF NEW-YORK. 
650. 3. PLATYOSTOMA HEMISPHERICA. 
Pl. LX. Fig. 2 a, b. 
JSuomphalus hemisphericus . Hall, Geol. Rep. 4th Dist. N. York, 1843, 109, fig. 1 & 2. 
Depressed globose; volutions about four; surface of volutions striated transversely, and, 
near the aperture, marked by some strong lines of growth. 
This species differs from the last, in the less rapid expansion of the last volution. It is only 
found in the condition of casts in the limestone, and therefore we know little of its original 
surface markings beyond the marks of striae preserved in the cast.. 
Fig. 2 a. View of the summit. 
Fig. 2 b. Front view. The shell is so much compressed, that the figures can give no true idea of 
the aperture. 
Position and locality. In the limestone above the shale at Rochester. 
Genus ACROCULIA. 
This genus, constituted by Prof. Phillips, corresponds in part to the Platyceras of Mr. 
Conrad ; and since this author has adopted the name AcRocuLrA, I shall use it in this place. 
Species of this genus first appear in the Niagara group, or possibly in the upper limestone of 
the Clinton group ; not a single species having been seen in lower silurian rocks. In the 
Niagara group we know but the two following species; and it is only at the period of the 
Delthyris shaly limestone of the Helderberg, that it becomes fully developed in a great variety 
of forms, and gradations of form which are unparalleled in any other formation. In the Oriskany 
sandstone, species of this genus become nodose, and in the upper limestones of the Helderberg 
there are others covered with long spines. In the succeeding Hamilton group it is not more 
frequent than in the Niagara group, one or two species being at present known. 
651. 1. ACROCULIA NIAGARENSIS (n. sp.). 
Pl. LX. Fig. 3. 
Apex involute, scarcely forming a volution, gradually expanding below with two or three 
longitudinal folds or undulations, transversely striated ; striae undulating across the elevations 
and depressions of the surface. 
The specimen is imbedded in shale, and only one side can be entirely seen. The shell is 
rounded with gentle longitudinal undulations, which do not rise into a ridge or angle. 
Position and locality. In the shale at Lockport. 
