288 



riixcyroLosT or ktw-toiii. 



650. 3. PLATYOSTOMA HEMISPHERICJL 



Pl- JLX. Fif . 2 a, h. 

 £luDlHfifcn?uii benuspfccricii^ . Hjt.i.1., GeoL Eep. 4Lb DisL N. Toilk, 164S, 109, ^. 1 k 2. 



Dejaewed globose; Tolations about four; surface of TolatioBs striated traorrerselj, and, 

 near the aperture, marked by some strong lines of growtb. 



This species differs from the last, in the less rapid eijiansaon of the last volution. It is only 

 found in the condition of casts in the limestone, and therefore ire know little of its ariginal 

 surface markings beyond the marks of stiiz preserved in the cast 



Fig. 2 ft. View of the summit. 



Fig. 2 I. Front view. The shell is so mucb compressed, thai tbe figures can give no troe idea of 

 the aperture. 



Position and locality. In the limestone above the shale at Rocbester. 



Giisrs ACROCULIA. 



This genus, constituted by Prof. Phtllips, corresponds in part to the TijiTTces^s oi Mr. 

 r.nx -a ATI ; and siDce this author bas adopted the name Ackoctlia, I shall use rt in liiis plac«- 

 Spedes of this genus first appear in the Xiagaxa group, or possflrfy in Ibe upper limestone of 

 the Clinton group ; not a single species baxing been seen in lower alurian rocks. Ln tbe 

 Xizisrara group we know but the two following species : and it is only at the period of the 

 Delthyris shaly limestone of Ibe Helderberg, that it becomes ftilly developed in a great variety 

 of forms, and gradations of form winch are unparalleled in any other formation- In the Qriskany 

 sandstone, spedes 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 spedes bfimg at present known. 



651. 1. ACSXXX'LIA MAGARENSIS («i.«p.5. 



PL.LX. 3RB.3. 



Apex involute, scarcely i'orming a -volution, gradually expanding below -witti two or three 

 longitudinal folds or tmdulations, transrersely striated; sIcte undulating across the elevations 

 and depressions of the surface. 



The specimen is imbedded in shale, and only cme side can be emtirely seen. The shell is 

 rounded with gentle longitudinal undulations, whidi do not rise into a ridge or angle. 



Position and locality. In the shale at Lockport. 



