98 THE NAUTILUS. 



junction of the lip with the body whorl is beneath the carina, in some 

 cases a short distance below, so that the carina is well marked on the 

 upper whorls, but usually the lip starts from just under the edge 

 of the carina, so that the side of the spire is almost straight, the edge 

 of the carina projecting but very slightly, if at all, beyond the surface 

 of the adjoining whorls ; the aperture is decidedly triangular, both 

 extremities being acutely angled, and the lip itself is sharply bent in 

 the centre where the carina appears. 



At about the beginning of the tilth whorl, the lip ascends and 

 crosses the carina, and from thence, until it finally disappears alto- 

 gether, the carina emerges from the upper part of the aperture. This 

 change in the relative position of the lip and carina induces a radical 

 change, not only in the shape of the lip, but of the whole shell. The 

 lip, in order to clear the carina. Incomes broadly rounded above and 

 curves in rapidly to meet the body whorl at almost a right angle, 

 forming a deep channelled suture, while the body whorl becomes 

 more ventricose, more or less shouldered, and rapidly increases in 



size. 



For about a whorl after the change takes place, the carina retains 

 its size and position, but after the sixth whorl is reached it gradually 

 diminishes and finally disappears entirely, although the body whorl 

 at all subsequent stages of growth is more or less flattened in its upper 

 part. 



With the disappearance of the carina, the shell rapidly assumes its 

 mature form and, with the usual erosion of the apical whorls, becomes 

 the short, stout, heavy specimen customarily seen in collections. 



NOTE ON THYSANOPHORA HORNII GABB. 



HY HENRY A. PILSBRY. 



My attention has been called by Mr. Geo. H. Clapp, of Pittsburg, 

 to the fact that some specimens of Thys. liornii show very fine deli- 

 cate cuticular riblets, more or less uneven at their free edges, and 

 running much more obliquely than the growth-lines, on the surface 

 of the last whorl. 1 This is usually obscured by the thin coat of earth 

 which encrusts the shell, and which it is evidently the function of 



'This structure has been lucidly described by Dall, Proc. U. S. Nat. Mus. 

 xix. 1896, p. 336. 



