Geol.— Vol. I.] SMITH— PL AC ENTICE R AS. 207 



On the fifth whorl the likeness to Hoplites becomes less, 

 and the characters of Placenticeras more strongly accentu- 

 ated; the central abdominal keel stands up almost as high 

 as those on the shoulders, while the sigmoidal ribs are no 

 longer so sharp as in the earlier adolescent stages. This 

 may be considered as the transition to maturity, although 

 the JJoplites characters do not disappear at once, nor do 

 the Placenticeras characters appear all at the same time ; 

 some, in fact, show even at the beginning of the adolescent 

 stage. A specimen illustrating the transition is figured on 

 PL XXV, fig. 8; it consists of four and a half coils, and 

 gave the following dimensions: — 



mm. 



Diameter 34 • 50 



Height of last coil iS . 00 



Height of last coil from the preceding i3-oo 



Width of last coil 9.50 



Involution 5 • 00 



Width of umbilicus 5 . 00 



P. californicuni might easily be confused with P. gitada- 



loufcB Stoliczka (1861) (not Roemer), of the Trichinopoly 



group, Upper Cretaceous ^ but differs from it in being more 



compressed laterally, in having rougher sculpture, and in 



lacking the heavy nodes on the sides below the abdoininal 



shoulders. The two species may be identical, but until we 



know the limits of variation of them, it would be unsafe to 



class them together. 



Placenticeras pacificum, sp. no v. 



Plate XXIV, Figs. 1-2 i; Plate XXV, Figs. 9-11; Plate XXVI; 

 Plate XXVII, Figs. 1-13; Plate XXVIII, Figs. 1-5, 



Placenticeras pacificum is large, discoidal, involute, and laterally com- 

 pressed, having the typical plate shape of the genus. At maturity the whorl 

 embraces somewhat more than one-half of the preceding; the breadth of the 

 whorl is one-fifth of the diameter, the height of the whorl is three-sevenths, 

 and the width of the umbilicus one-fifth. The body-chamber is about two- 

 thirds of a revolution in length. The abdomen is narrow, flattened, slightly 

 concave, bounded by a row of elongated knots forming rough keels. Up to 

 diameter of about one hundred millimetres, five and a half coils, the shoulder 



1 Stoliczka's species is probably not identical with Roenier's P. gnaciatoupce, from the 

 Lower Cretaceous of Texas, zone of Mortontceras rostratuvt. 



