entire shell: >mu- deep, tin- Inl.c- not rounded hut bending at almost a right angle 

 in tlio middle: inner lip slightly re flex ed and thickened on each side; test thin, 

 rnmpoM'd apparently <>f two layers, tlie inner nacrous. Surface of specimens marked 

 l.y more or less nl.M-ure, lines of growth. These are usually the strongest on the dor- 

 suin, where they may l>o seen also on some casts of the interior. The latter, however, 

 are in most cases quite smooth and without a trace of markings. External 

 layer of shell not seen in good condition, so we cannot say positively, though 

 we suspect it to be so, that it was marked near the umbilicus, as in the following 

 species, hy tine granulose stri.r. Might of shell 20.5 mm.; median hight of aperture 



iin.: width of same 1'.' nun.: \\idth of inner volution 7 mm.; depth of sinus 8mm. 

 In two other specimens, one very large, these dimensions are respectively, 24, 10, 23, 

 7 and ( .i nun., and 30, 13.5, 27, 8 and 12 mm. 



This species is often met with in collections under the name of Bellerophon 

 liilotxitns, hut it is really quite distinct, as anyone may see by comparing the 

 accompanying figures of that species with those of B. rectanyularia given on plate 

 I. Mil. 



L' 



1. A and H. tw. vn-wsnf l- r ,,t,r<irthi.i '.i/'.W.i S.wcrby sp., and C, two Tlewi of BelUropkon 

 1 'aradoc saml-tunr, Kn^-Und. Copies of the original figures of these species oo 



I-l.il' " -.-t.-iii." 1838. 



It will be noticed that />' hilnhuiHx, or 1'rutnimrtkia bilobata as it should now be 

 railed, is a more ventricose shell, causing the back to be fuller and much more 

 broadly rounded, while the apertural lobes, are, as usual, rounded in front instead 

 of angular. The rectangular outline of the apertural lobes which has suggested the 

 specific name, dMinguMie^ /'. ncfmnjuliiris from all the other forms referred to 

 Prolotcarthiu. Indeed, it is a peculiarity that is not possessed by any other species 

 of the whole suborder. 



We have before us three specimens of a variety of this species from the Trenton 

 limestone at Trenton Falls, New York. s.. far as this material at hand shows, the 

 variety differs only in having the apertural lobes less angular than usual. We have 

 also a number of internal casts from the Black River and Trenton shales and 



