GASTROPODA. 915 
Bellerophon troosti.] 
the total absence of revolving surface lines. Within its own family, the 
Bellerophontide, Bellerophon has not the revolving sculpture of Bucanopsis, nor the 
extremely expanded aperture of Patellostium, nor the revolving folds of the inner 
lip which characterize Huphemus, while neither Mogulia nor Warthia have a distinct 
slit-band. 
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 less strong callosity on the iuner lip, and (5) a well developed, generally raised, 
slit-band terminating anteriorly in a short median emargination or slit in the outer 
lip. 
The genus might be 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 contortus group of Koken, we have already distinguished 
it as a separate genus under the new name Megalomphala (see p. 850), this type 
belonging in our opinion to the Bucaniide rather than the Bellerophontide. 
Ten of the fifteen American Lower Silurian species retained as true bellerophons 
are figured 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 
first time and all are more or less closely and obviously related. 
BetiERopHON TROOosTI (D’Orbigny) Safford. 
PLATE LXIV, FIGS. 1—5. 
Bellerophon troosti D’ORBIGNY, 1840, Cephalopoda, p. 206; as figured by Safford, 1869, Geol. of Tenn., : 
: pl. a@, figs. 4a—4d. 
Shell beneath the medium size, rarely exceeding 17 mm. in diameter; somewhat 
transverse, the width of the aperture being greater than the hight; whorls rather 
broad, inflated, though somewhat depressed on each side of the prominent dorsal 
carina; on the sides they are strongly convex, rounding into the small but deep and 
constantly developed umbilicus. In the adult shell the carina is rounded, but in 
young specimens the summit is flat or slightly excavated, forming a distinct slit- 
band on which lunule 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 thickened and much 
expanded laterally and horizontally, the inner edge forming a thick low biconcave 
ridge with a rounded central prominence; the latter constricted within the mouth 
and continuing inwardly as a distinct ridge; latero-ventral angles turned backward, 
