2o6 CALIFORNIA ACADEMY OF SCIENCES. [PROC. 3D Ser. 



both to the group of H. interrufttis , but the late adolescent 

 stage of P. pacificum corresponds closely to H. splendens, 

 while P. californicuni corresponds to the rougher and more 

 evolute species, and retains many cosmoceran characters 

 lacking in the other form. 



As the adolescent period advances the resemblance to 

 Hoplites diifrenoyi d'Orbigny, of the Gault of Europe, 

 becomes marked, although P. calif ornicum differs from the 

 European species in having the ribs finer, more numerous, 

 more sharply bent on the sides, and in having the rows of 

 knots on the abdominal shoulder keels closer together, and 

 weaker. There is also a rather faint second row of knots 

 high up on the sides, where the ribs fork at the sharp for- 

 ward bend; this is a cosmoceran character lacking on ^. 

 dicfrenoyi. The ribs start out in pairs from a knot on the 

 umbilicus, and between each pair there is usually a single 

 rib intercalated. The rows of knots on the umbilicus, on 

 the sides, and on the abdominal shoulders, are stronger at 

 diameter of fifteen milHmetres than at maturity, showing 

 this to have been characteristic of the immediate ancestor 

 of the genus. The relative measurements agree exactly 

 with H. dufrenoy?\ except that on the Californian species 

 the whorl is broader, and the cross-section polygonal. 



The adolescent stage of P. californicum, PI. XXV, 

 figs. 6 and 7, at about four coils, gave the following 

 dimensions: — 



mm. 



Diameter 22.00. .. .1.00 



Height of last whorl 10 . 50 .... o . 48 



Height of last whorl from the preceding . . 8 . 25 o . 37 



Width of last whorl 8.00 0.36 



Involution 2.75... .0.11 



Width of umbilicus 6 . 30 . . . o . 28 



Compare with these the almost identical measurements of 

 H. diifrenoyi, at the same stage of growth: — 



mm. 



Diameter 21 . 50 .... i . 00 



Height of last whorl 10.00. . . .0.47 



Height of last whorl from the preceding . 7. 50.... 0.35 



Width of last whorl 6 . 50 .... o . 30 



Involution 2.75 o.ii 



Width of umbilicus 6.00 0.28 



