PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 141 
fossil shells, not any of them were sufficiently well preserved to indicate 
even their generic relations. No satisfactory information has been 
obtained concerning any geological observations that may have been 
made in that region, which might convey a knowledge of the geological 
age of the strata of the locality from which the fossil in question was 
obtained, and I am therefore under the necessity of relying wholly 
upon the testimony afforded by the fossil itself. The genus to which [ 
have referred it has hitherto been known only in rocks of Cretaceous 
age; and there appears to be no good reason to doubt that the strata 
from which this Mexican shell was obtained belong also to that period. 
Genus TYLOSTOMA Sharpe. 
TYLOSTOMA PRINCEPS (sp. nov.). 
(Plate II, figs. 1 and 2.) 
Shell very large, general form rhombic-ovate, inflated; spire moder- 
ately extended; volutions five or six, convex, having an ill-defined nar- 
row shouldering at the distal or upper portion, adjacent to the suture; 
umbilicus none, suture impressed; aperture ovate-semilunate, large, its 
length equal to more than two-thirds the full length of the shell; outer 
lip forming an approximately regular curve from near the suture to the 
anterior portion of the aperture, which, although broad, is somewhat 
produced; margin of the outer lip only slightly sinuate; inner lip bear- 
ing a broad, moderately thin callus, its outline somewhat strongly sin- 
uate and its margin narrowly flexed along its anterior portion. 
Surface marked by the ordinary lines of growth. 
Length from the apex to the front margin of the aperture, 220 milli- 
meters; greatest breadth, 160 millimeters; length of aperture, 150 mil- 
limeters. (Museum, No. 8864.) 
This is much the largest fossil gasteropod that has ever been found 
in North American Mesozoic strata; and it is excelled in size by only 
comparatively few of its class that have since existed. 
It has much the general aspect of a Lunatia, but it is referred with- 
out much hesitation to the genus Tylostoma Sharpe. This last-named 
genus is regarded by some malacologists as having affinities with the 
Tectibranchiata, near Pterodonta ; but I agree with Stoliezska and Zit- 
tel in referring it to the Pectinibranchiata, and placing it near Lunatia 
in the Naticide. It is true that al! the characteristics of Tylostoma, as 
enumerated by Sharpe and characteristic of most if not all the species 
which have been referred to that genus, are not clearly observable upon 
the only example of this species that has been discovered; but being 
plainly without an umbilicus, or any umbilical perforation, in connection 
with its other characteristics, it cannot be referred to any other recog- 
nized genus of the Naticide. The condition of our example is not such 
as to show clearly whether or not the outer lip was thickened at the 
time of the death of the mollusk. 
