﻿445 



EURYSTOMITES UNDATUM. 



Nautilus undatus, Hall {Pal. New York, i, p. 52, PL xiii and 

 xiii bis). 



Loc, Poland, Herkimer county, N. Y., Black River Limestone. 



PI. v, Figs. 1, 2. 



This species has much broader whorls in the young than in 

 Eurystomites of the Quebec faunas. The position and size of the 

 siphuncle, the large umbilical perforations and the sutures and the 

 flattened abdomen of the adult stage and comparison with the 

 heavily costated forms like Discoceras antiquissimum and others 

 show that this shell is probably a member of the genus Eurysto- 

 mites. 



The nepionic stage has a very large umbilical perforation and in 

 correlation with this the whorl has a rounded dorsum at this age. 

 The metanepionic substage has a broadly elliptical form and sub- 

 ventran siphuncle; the paranepionic, on account of the rapid trans- 

 verse growth, has a subdigonal whorl, as shown in Fig. 2, PL v. 

 The shell does not increase so fast transversely in the neanic stage 

 and the whorl becomes rounder, a slight contact furrow appearing 

 when the whorls come in contact, and the living chamber at this 

 age is less than one-half of a volution in length. Light costations 

 also appear in this substage, but the nepionic whorl is smooth with 

 the exception of strong lines of growth. The abdomen becomes 

 flattened in the paraneanic substage, and the sutures show slight, 

 ventral lobes and very slight lateral lobes. 



The siphuncle is propioventran in the paranepionic substage and 

 neanic stage. The costations become stronger in the paraneanic 

 substage but are reflected on casts only to a very slight extent. 



The young are, however, quite variable, and the figures, PL v, 

 give probably an extreme form so far as the retention of the broad 

 subdigonal form is concerned. In other varieties or species, for I 

 think there are several species usually placed under this name, the 

 sides and venter become slightly flattened in the metaneanic sub- 

 stage, or even before this. In the specimen figured this change 

 had not yet taken place, although the shell was in the metaneanic 

 substage. 



The aperture in this specimen flared out laterally, but is removed 

 in the section Fig. 2, PL v. 



Slight, foldlike costoe are better developed in the ephebic stage, 



