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of the Trias, it is apparently shorter in duration than in the gener- 

 alized and less complex organization of Tirolites if one can judge 

 by the simple characters of the ephebic stage. 



In the Jura and Cretaceous, among the Ammonitinae and Lyto- 

 ceratinae, typical Ammonoids with more highly specialized struc- 

 tures than any Triassic shells, the primitive characters of this sub- 

 stage are, as one can read in Branco's drawings and to a less extent 

 in mine, still more limited in extent, being confined as a rule to a 

 few sutures or to one, and finally, in many forms they are obliter- 

 ated altogether. That is to say, the divided ventral lobe encroaches 

 upon and finally obliterates the intermediate stage so that the meta- 

 nepionic substage, which begins with the third septum and micro- 

 siphon, is wholly changed in the aspect of the sutures. In other 

 words, the undivided ventral lobe of the Nautilinidae has been re- 

 placed in this substage by the divided ventral of the Primordialidae 

 which appears in the suture of the second septum. 



This is also, like the preceding, an excellent example of what is 

 meant by the law of tachygenesis, the earlier inheritance through 

 the crowding back and replacement of distal by proximal genetic 

 characteristics. 



Fig. 3, PL iv, shows the prolonged duration of the nautilinian 

 characteristics in this substage in second, third and fourth septa of 

 Vermiceras (Arietites) spiratissimttn of the Lower Lias, the decided 

 change to a divided ventral and two lateral lobes not coming in 

 until the seventh suture. 



Fig. 7, PL iii, shows the section of Deroceras planicosta of the 

 Lower Lias and the delayed approximation of the siphuncle to the 

 ventral side. Fig. 7 shows the primitive structure of this organ in 

 the earlier substages, and the figures from Branco show the duration 

 of characteristics to be in correlation with these primitive charac- 

 teristics. 



Fig. 7, PL iii, shows the structure of the siphuncle in the meta- 

 nepionic substage. The transitional aspect of the second septum 

 can be observed in Figs. 6 and 7 of the same plate. This is a 

 direct reference, as I shall show in another paper, to the similar 

 structure of the ephebic siphuncle, and also the swollen aspect of 

 the early stages of the siphuncle in the Endoceratidoe, although in 

 some species of this family as many as six funnels may take part in 

 the construction of the swollen apical end of this organ. These 

 facts are also in direct correlation with the more specialized and 



