Geol.-Vol. I.] SMITH— PLACENTICER AS. 20$ 



Larval and Adolescent Stages. 



The earliest stages of Placenticeras californiami could 

 not be observed on the specimens at hand. The smallest 

 specimen obtained, figured on PI, XXV, figs, i and 2, was 

 in the perisphinctoid stage, and very like the corresponding 

 stage of Placenticeras -pacijiciim, except that the shell is a 

 little rougher, and somewhat more evolute than on that 

 species; it consisted of two and three-eighths coils, and 

 had the following dimensions: — 



mm. 



Diameter . 3-32 



Height of last coil i • 44 



Height of last coil from the preceding i.ii 



Width of last coil i .33 



Involution 0.33 



Width of umbilicus i . 16 



At about two and a quarter coils the constrictions cease, 

 the ribs end in knots on the abdominal shoulders, and the 

 cosmoceran stage begins at diameter of about four milli- 

 metres, just as in P. -pacijicuni, but the sculpture is rougher, 

 and the shell more robust. At about three coils and diam- 

 eter of eight millimetres the sculpture and shape of the shell 

 resemble strongly Hoplites tuberctilatus Sowerby, of the 

 Gault; this specimen, figured on PL XXV, fig. 3, gave the 

 following dimensions: — 



mm. 



Diameter 8 . 00 



Height of last coil 4.10 



Height of last coil from the preceding 2 .80 



Width of last coil 3 . 30 



Involution i . 30 



Width of umbilicus i . 80 



At three and five-eighths coils, diameter 14 mm., the 



resemblance to the group of Hoplites interriipttis has become 



so striking that the young shell might well be taken for an 



adult of that group, were it not for the fact that this entire 



group of Hoplites was either extinct at this time, or at least 



had changed entirely into other genera. This specimen 



is figured on PI. XXV, figs. 4 and 5, to illustrate the most 



typical Hoplites stage of growth of the species, and to show 



the striking difference between P. californicuin and P, 



pacijicum at this stage. Both correspond to Hoplites, and 



