2o8 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



keels are rather finely notched by the ends of the lateral ribs; at this stage 

 the coarse elongated knots of Placenticeras appear, and deep undulations 

 cross the abdomen; a central keel, which has persisted up to this stage, be- 

 comes obsolete. The sides are ornamented with rather coarse sigmoidal 

 ribs, bundling on umbilical knots; these show on the cast even more strongly 

 than on the shell. The outer shell has numerous sigmoidal stria; covering the 

 ribs and the interspaces. The umbilicus is moderately narrow, and the 

 shoulders angular, becoming more so as age advances. At five and two-thirds 

 coils the umbilicus is one-sixth of the diameter, while at six and one-third 

 coils it widens out to one-fifth, this change probably indicating old age, for 

 the slackening of the increase of height of the body-chamber shows a decrease 

 in growth force. 



The septa consist of an abdominal lobe with a pair of long branches, and 

 seven lateral lobes, all finely digitate, and rather narrow. The saddles are 

 broader and deeply divided by narrow secondary lobes. 



Douville (1890, p. 288) says that in Placenticeras the 

 first, second, and third lateral lobes are probably formed 

 out of divisions of a primary lobe, and that the fourth is 

 really the second primary. But the development of this 

 species shows that the first lateral lobe is developed out of a 

 division of the primary lateral saddle, and that the second, 

 third and fourth lobes are developed out of notches in the 

 primitive first lateral lobe. There are also three auxiliary 

 lobes on the sides, and one on the umbilical shoulders, 

 growing simpler as the umbilicus is approached. This is a 

 common and well known fact, ascribed by Jackson (1899) 

 to the principle of locaHzation of stages of growth, by which 

 more primitive characters are preserved in the dorsal and 

 umbilical portion of the shell. At early maturity these 

 lobes are arranged in a wide backward-pointing curve, but 

 in later growth this curve is not nearly so pronounced. The 

 development of the septa, as shown in the adolescent 

 stages, makes it clear that the three chief lateral lobes are 

 merely modifications out of the three points of the tri^enid- 

 ian primitive lateral lobe. This explains the arrangement 

 in a curve, and also the straightening out of this curve as 

 full maturity is approached. This suggests a probable 

 explanation for the large number of lobes found in many 

 ammonites, such -a.^ Placenticeras and Sphenodisciis, although 

 it is not yet known that the development of the latter genus 

 takes place in this way. The arrangement of the lobes in 



