OASTROPOI'A 



in- 



the total absence of revolving -urface lines. Within its own family, the 

 lieller-'plifiHt > ."Aon has not the revoh ing sculpture of hn,-,, n^/mis, nor the 



i \tremely expanded aperture of PofottMfNMM, nor the revolving folds of the inner 

 lip which characterize Kuphttnus, while neither Moyulin nor Wnrthiit have a distinct 



The principal distinctive features of the genus are: (1) the absence of all kinds 

 of sculpture save the more or less strongly developed lines of growth, (2) the small 

 or entirely closed umbilicus, (3) the moderate expansion of the aperture, (4) a more 

 or le >trong callosity on the inner lip, and (5) a well developed, generally raised, 

 si it -hand terminating anteriorly in a short median emarginatioo or slit in the outer 



1'P. 



The genus might he divided into several subordinate groups none of them, 

 however, seeming of more than doubtful utility. One, Waagenella, including a few 

 Carboniferous species, is distinguished by a definite callosity in the umbilical region. 

 Regarding the Bellerophon cuntortus group of Koken, we have already distinguished 

 it as a separate genus under the new name .Uy/.i/w/*//// (see p. 850), this type 

 belonging in our opinion to the liuranin/n rather than the IMIer]tlionti<l<t . 



Ten of the fifteen American Lower Silurian species retained as true bellerophons 

 are tigured and described in this report, giving a very good idea of the genus as 

 represented in this part of the Paleozoic rocks. Most of them are described for the 

 rir-t time and all are more or less closely and obviously related. 



BKLLKROPHON TROOSTI (D'Orbigny) Sajford. 



PLATE I.X IV. KIO8. 1-4. 



BeUervpAon tmoiti I M. MO, Cephalopoda, p. 206; as figured by Safford, 1869, Geol. of T. Tin.. 



pi. o, figs. 4 4d. 



Shell beneath the medium size, rarely exceeding 17 mm. in diameter; somewhat 

 transverse, the width of the aperture being greater than the bight; whorls rather 

 l'iua'1. inflated, though somewhat depressed on each side of the prominent dorsal 



.na; on the -id-- they are strongly convex, rounding into the small but deep and 

 constantly developed umbilicii*. In the adult shell the carina is rounded, but in 

 young specimens the Mimmit is Hat or slightly excavated, forming a distinct slit- 

 band on which lunuhr are either not preserved or were originally very faint. Aper- 

 ture greatly expanded laterally, outer lip sharp and thin, with a deep and rather 

 narrow subrectangular central emargination: inner lip tliickcne-l and much 

 expanded laterally and horizontally, the inner edge forming a thick low l>iconcave 

 ridge with a rounded central prominence; the latter constricted within the mouth 

 and continuing inwardly as a di-tinct ridge; latero-ventral angles turned backward. 



