288 



PALAEONTOLOGY OF NEW-YOUK. 



650. 3. PLATYOSTOMA HEMISPHERICA. 



Pl, LX. Fig. 2 a, b. 

 Euomphalus hemisphericus. Hall, Geol. Rep. 4lh 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 strise 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. 

 CoNKAD ; and since this author has adopted the name Acroculia, 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 Heklerberg, 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 ; striee 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. 



