THK NAUTILUS. 15 



especially conspicuous just below the suture ; on the uppermost of 

 the remaining whorls are faint nodes, about twenty to the whorl ; 

 aperture rather narrow ; posterior sinus shallow, rounded ; lip 

 slightly produced below the sinus, as shown by the growth lines ; 

 two-thirds of last whorl broken away ; canal rather wide ; pillar 

 solid, curved, a somewhat prominent, smooth swelling above ; body 

 whorl eroded in front of the aperture, with a wash of callus striated 

 in lines parallel to the lip. 



Length of shell 59, of aperture and canal 30, of body whorl 41 ; 

 max. diam. 20.5 mm. Divergence 30. These measurements are 

 taken from the broken shell. The spire was probably about 4 mm. 

 longer and the total length of the perfect shell could not have been 

 less than 75 mm., with 7 or 8 whorls. From the Pliocene of Santa 

 Monica, California. 



Genota riversiana is the narrowest form of this subgenus found in 

 California. Only an occasional, greatly produced G. carpenteriana 

 Gabb can compare with it in this respect. The narrow form, the 

 prominence of the revolving sculpture and the obtuse angle, behind 

 the middle of the whorls, well characterize this species. In G. 

 cooperi Arnold and G. tryoniana Gabb the angle is sharper and in 

 front of the middle of the whorls. The principal revolving threads 

 in the present species are more conspicuous and more nearly equal 

 than in G. cooperi, of which a fine specimen from the Pleistocene of 

 Santa Monica is before the writer. The latter species is the most 

 sharply sculptured of those hitherto described. Form and sculpture 

 at once separate G. riversiana from G. carpenteriana Gabb, and still 

 more widely from G. stearnsiana Raymond, which is the shortest and 

 widest species of the subgenus. 



It gives me great pleasure to dedicate this beautiful species to the 

 discoverer, Professor J. J. Rivers, who has made a large and most 

 interesting collection of the Pliocene and Pleistocene fossils found in 

 the vicinity of his home. 



The list of subgenus Genota will now be as follows : 



1. Genota carpenteriana Gabb, Late Tertiary to Recent. 



2. " tryoniana Gabb, Late Tertiary to Recent. 



3. " cooperi Arnold, Quaternary. 



4. " stearnsiana Raymond, Recent. 



5. " riversiana Raymond, Late Tertiary. 



It is a very compact group. No. 2 is scarcely more than a variety 



