«9« PALAEONTOLOGY OF NEW-YORK. 



GASTEROrODA OF THE LOWER HELDERBERG GROUP. 



Thb species of this class of fossils are much more numerous in the Lower 

 Helderberg group, than in any of the preceding or succeeding palaeozoic 

 periods below the Coal measures. They include, moreover, the greatest 

 extremes, as well as a great variety of forms. We have the slender spiral 

 shells with numerous closely arranged volutions, as in Murchisonia and 

 LoxoNEMA, and the broad ovoid forms with one or two volutions at the 

 apex ; the slender forms with the volutions free, and the conical or 

 broadly depressed-conical forms which are straight or nearly straight, 

 having no evidence of convolutions whatever, or with a slight arcuation 

 at the apex. 



Notwithstanding all this variety of form and degree of development in 

 the spire, there is but a single nodose species known to me in this period ; 

 and in this one, the nodes are rather like transverse interrupted ridges*. 

 The surfaces of many species are spirally or longitudinally ridged, and 

 often transversely or concentrically lamellose or lamellose -striate, while 

 a few forms are strongly cancellate. 



Two species of Euomphalus are known in the strata of this age, and a 

 single species of Bellekophon or Bucania. The few species of Loxonema 

 and Murchisonia are unfortunately in the form of casts, and their study 

 is thus rendered unsatisfactory. 



A large number of the forms are such as are at the present time re- 

 ferred to the Genus Capulus of Montfort ( Pileopsis of Lamarck), with 

 which Acrocuua of Phillips and Plattceras of Conrad are made synony- 

 mous. For certain other forms among these shells, Mr. Conbad proposed 



• The earliest nodiferons or properly spiniferous form of gastcropod occurs in the Orisliany sandstone, 

 the casts of which are strongly nodose, and the shell ornamented with strong spines. The spine-l<earing 

 gatteropods are common in the Upper Uelderberg group, and are Icnown in the Uaniilton group and in the 

 Carboniferous limestones. 



