Geol.-Vol. I.] SMITH— PLACENTICERAS. 21 7 



intermediate ribs on the sides and abdomen. It should be 

 noted that the development of the septa has not kept pace 

 with the other characters, for in small adolescent stages of 

 ammonites, on account of their minute size and the thickness 

 of the shell, it is physically impossible for the septa to have 

 the complexity of the corresponding adult genus. Another 

 reason is that the retardation in development of the septa, 

 which is distinctive of Placenticeras, begins to show itself 

 here. This retardation becomes more pronounced as the 

 stages advance, so that it becomes more than mere unequal 

 acceleration of characters, for in the septa the HofJites 

 stage of development is never reached, the differentiation 

 of the first lateral lobe into three secondary lobes being 

 already complete. This differentiation shows itself quite 

 distinctly in the perisphinctoid stage. The other characters 

 are not affected by retardation to nearly so great an extent 

 as the septa, or it may be more correct to state that retarda- 

 tion can not be detected in the other characters. 



A specimen of two and a-quarter coils, figured on PI. 

 XXIV, figs. 14 and 15, gave the following dimensions: — 



mm. 



Diameter 3 . 50 



Height of last whorl i . 63 



Height of last whorl from the preceding i . 36 



Width of last whorl i . 40 



Involution 0.27 



Width of umbilicus i . 06 



In everything but the comparative simplicity of the septa 

 this stage is distinctively perisphinctoid, but cannot be cor- 

 related with any particular genus of that group, especially 

 in view of the fact that according to the researches of Buck- 

 man, quoted above, Perisfhinctes itself is a degenerate of the 

 stock of the Stephanoceratidee. The perisphinctoid stage 

 lasts up to two and a-quarter coils, diameter about 3.50 mm. 



Metaneatiic — Cosmoceras Stage. — In this stage the ribs 

 no longer cross the abdomen, but end in tubercles on the 

 abdominal shoulders, forming well defined shoulder keels, 

 with a furrow between them. The varices and constric- 

 tions cease abruptly with the beginning of the stage, at 

 about two and a-quarter coils. The ribs become faint on 



