286 



PALEONTOLOGY OF NEW-YORK. 



GASTEROPODA OF THE NIA(JARA GIIOUP. 



The difficulty of finding reliable characters in the shells of Gasteropoda, is greater than in 

 any other family of fossils. The form is liable to great variation, both from natural and artificial 

 causes. The surface markings are vaiiously developed, or developed in very different degrees, 

 and these characters are likewise the first to suffer from abrasion or solution. There are otljer 

 variations of character dependent upon age, which modify the form and proportions as well as 

 surface characters. Under such circumstances the number of nominal species can scarcely fail 

 of being much increased ; and where a few specimens only fall under observation, it is often 

 impossible to connect them, from the want of others showing intermediate characters. 



The following specimens figured on Plate 60, fig. 1 a - y, present, in tlieir extremes, characters 

 sufficiently different to authorize the establishment of species ; but after a comparison of these 

 and numerous other specimens, I am satisfied that they all belong to a single species, presenting 

 numerous gradations of character naturally, and other phases induced by accidental causes. 



Genus PLATYOSTOMA (Conrad). 



" Shell subglobose ; spire short ; aperture very large, suborbicular, dilated ; labrum joining 

 " the body-whorl at right angles to the axis of the shell." 



This description alone may not be sufficient to separate species of this genus from other 

 genera. The larger proportion of the specimens of those species given by Mr. Conrad show 

 a lower spire, either from compression or other causes ; and they approach so nearly to those 

 fossils referred to Natica, Nerita and Naticopsis, and sometimes to Euomphalus, that we 

 have no sure guide to determine us in the reference of species. 



The presence of a sinus in the outer lip has been regarded as a character sufficient for the 

 reference of species to Pleurotomakia ; and yet in other respects the shells may vary to 

 extreme degrees, and the form and character of this sinus is equally variable and extreme. In 

 some of the following specimens there is a broad shallow sinus in the outer lip, and an arching 

 of the striae to correspond therewith. Sometimes the back of the shell is marked by a band 

 more or less elevated and conspicuous, extending from the sinus nearly to the end of the first 

 volution. In other specimens there is almost no evidence of the sinus, and scarcely a visible 

 arching of the strioe. In one specimen the band and arching of the striffi is visible in the upper 

 part of the last volution ; while for the last half inch before reaching the aperture, the striae 

 are not visibly undulated, nor is there any band or elevajtion. This character, therefore, I infer 

 to be of less importance than is usually supposed ; and if it may be present or absent in the 

 same species, it is clear that this alone can not be relied upon for generic distinction. 



