OF SOUTHERN INDIA. 37 
of Tylostoma, described and figured by Sharpe, and also in several of the species 
figured by Pictet (loc. cit. pl. LXXIII.). Two of our species show the ex- 
pansion equally distinctly. It appears, that this expansion of the outer lip is 
never wanting in perfect specimens, but on the other hand it is almost certain, 
that it attains its proper size only after a certain stage of growth of the specimen. 
Before the expansion takes place the outer lip is provided internally with a strong 
elongated varix or fold, which narrows the space of the aperture laterally to a great 
extent. This fold or tooth is either smooth and simple, or it has the internal edge 
more or less crenulated, as is clearly exhibited in the figures of Plerod. inflata and 
intermedia, D’Orbigny, Tylostoma ovatum, Sharpe, and others. This inner fold or 
varix of the outer lip varies extensively in its size. In some specimens it is tooth- 
like and placed posteriorly, or in the middle, or nearer to the anterior termination of 
the outer lip; in other specimens it is much more prolonged and extends from the 
anterior extremity nearly to the posterior suture, that is, across the entire breadth of 
each whorl. There is every variation to be observed in the different species as 
regards the size and the strength of this thickening on the outer lip. In young 
specimens it is less or not at all developed, just as is the expansion of the lip itself, 
In this state of age the shell of Pterodonta cannot be better compared with any 
other genus than with Priamus (Halia, Risso). It would be interesting to compare 
with these forms better preserved specimens of D’Orbigny’s Globiconcha. From 
the comparison of a large number of different forms, it appears, that the internal 
varices had occasionally been again absorbed during the succeeding growth, and that 
only some of the last ones, or actually only the last near the margin of the 
aperture, remain unaltered; cast-specimens of shells are especially important for 
studies of this kind. 
Tt will be readily seen from these notes, that the number or even the existence 
of the varices or their impressions on the upper whorls cannot be regarded as being 
of very much value generically or perhaps even specifically. When the varix is 
tooth-like and anteriorly placed there will never be a trace of an impression seen 
on the upper whorl, because their lower portions are always covered by the suc- 
ceeding volutions. This is, for instance, the case in the typical Pterodonta inflata, 
D’Orb. The impressions of the varices do, however, exist, in this species as I had 
occasion to observe on a few well preserved casts, which I broke up for that purpose. 
In cases where the varices are placed posteriorly, or where they extend close to 
the posterior margins, their impressions will be clearly observable on all the upper 
whorls, unless the varices had been previously absorbed. Instances of all these 
variations may be seen by a comparison of the figures of D’Orbigny’s Pterodonta 
and Sharpe’s and Pictet’s Tylostoma, the identity of which can scarcely be doubted 
from all that is known of their structure at the present. 
Having thus treated at length the generic characters and their variations in 
Pterodonta, it remains only to say a few words as regards the classification of the 
genus among the numerous families of the order PRosoBRANCHIA. 
Excluding a few fusiform species, to which we shall draw special attention 
subsequently, we have in Pterodonta, smooth shells of an oval or conical shape, with 
L 
