56 NEW YORK STATE MUSEUM 



ribs disappear entirely, so that in such individuals the peristome bears a 

 very distinct ornamentation. Casts of the interior surface of the shell 

 demonstrate that the latter was almost entirely smooth, and bore no other 

 traces of the highly sculptured surface ornament than occasional very faint, 

 low, revolving undulations or obscure indications of the transverse ribs. 



The transverse folds by which the surface sculpture is crossed, are 

 apparently the results of the repeated production and absorption of the 

 expanded mouth. The development of the latter at more or less frequent 

 intervals is a noteworthy difference from the earlier Siluric forms referred 

 by Ulrich to the genus Salpingostoma F. Roemer, 1 the fundamental distinc- 

 tion, however, being in the character of the perforations on the slit-band, 

 which in Trematonotus are elliptic, with everted margins, while in Salpin- 

 gostoma the perforation is a single long and continuous but inclosed slit. 



In T. alpheus the perforations are located on an elevated, narrow, 

 dorsal keel, which does not extend on the peristome, but is there followed 

 by a depression extending to the margin of the outer lip. To the keel cor- 

 responded a deep groove on the inside of the shell. The groove and per- 

 forations disappear where the inner lip of the peristome reclines on the 

 penultimate volution, and closes the perforations. The number of perfora- 

 tions left open varies from six to nine. 



Dimensions. A large specimen with gerontic characters measures from 

 the outer edge of the aperture to the dorsal side of the early part of the 

 ultimate volution, 75 mm. The major diameter of the aperture was about 



57 mm and the vertical diameter of the shell 51 mm. 



Observations. The genus Tremanotus (recte Trematonotus Fischer) 

 was erected by Hall as above cited for this species, the original specimens 

 being from the Chicago limestone. Professor Hall had earlier described 

 from the Guelph formation at Gait Bucania angustata,* a species 



1 The type of this genus, S. m ac r o s t om a, is a middle Devonic shell. It is yet to 

 be demonstrated that the Trenton shells which have been referred to it are congeneric 

 with this species. 



2 Pal. N. Y. 1852. 2:349, pi. 84, fig. 6a, b. 



