176 RALPH TATE. 
Biconic ; keels two, elevated; the posterior keel overlapping the 
suture, sharply crenate on the edge ; the peripheral keel is triplic- 
ate and granose-dentate, anterior to it on the body-whorl are 
largely elevated encircling ribs (about twelve) narrower than the 
sulci which are transversely wrinkled and produce crenatures as 
they pass over the ribs. Outer lip with a deepish and broad slit 
at the periphery, anterior to which it is very strongly arched in 
marginal outline and declivous towards the columella contracting 
the aperture to narrow-elliptical. Length 35, greatest width (at 
one-third of the whorl from the aperture) 17, height of last whorl 
24-5 mm. Localities—Eocene: Muddy Creek, Gellibrand, 
Fyansford, Mornington. 
G. angustifrons is a variable species, in respect of length and 
details of ornamentation, but my description applies to the more 
commonly prevailing form, which is fairly intermediate between 
the extremes of variability. 
Family CerITHIuD®. 
Genus Diastoma, Deshayes. 
This name is in substitution for Mesalia, to which I referred 
our only species, M/. Provisi (inedit.), because congeneric with 
Mesalia melanoides, Reeve, Icon. Conch., 1849. My valued cor- 
respondent M. Cossmann, to whom the fossil was sent under the 
above name, informs me that it is a Diastoma, from him I have 
received examples of several species of Diastoma and Mesalia from 
the Parisian Eocene. This material permits me to affirm that I. 
Provisi, mihi, and Jf, melanoides, Reeve, are congeneric with D. 
costellatum, Lamk.; whilst Mesalia sulcata, Lamk. (non sulcata, 
Gray, = brevialis, Lamk.), is of a totally different type. Diastoma 
simulates Mesalia, but the latter has a sinuated outer lip, whilst 
the spiral carination of the columella of Diastoma is quite a 
different feature from the slight twist of the columella-margin of 
Mesalia; moreover Diastoma is more or less variced. Reeve in his 
figure and description of Jf, melanoides omits the slight varices 
on the posterior whorls ; at any rate the examples of this species 
