PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 141 



fossil sliGlls, not any of tlieui were sufficiently well preserved to iiulioate 

 eveu 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 I 

 have referred it has hitherto been known only in rocks of Cretaceous 

 age; and there aj)pears 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, land 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 

 l)roduced; margin of the outer lip only slightly sinuate; iuner lip bear- 

 ing a broad, moderately thin callus, its outline somewhat strongly sin- 

 uate and its margin narrowlj^ 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. 88G4.) 



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 Liinatia, 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 Stoliczska and Zit- 

 tel in referring it to the Pectinibranchiata, and placing it near Lunatia 

 in the Naticid?e. It is true that all the characteristics of Tylostotna, 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, ft cannot be referred to any other recog- 

 nized genus of the Naticidre. 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 moUusk. 



