Geol.— Vol.. I.l SMITH— PLACENTICER AS. 227 



stage after the correlative genus, instead of using an arbi- 

 trary and artificial nomenclature such as has been adopted. 

 When ontogenic stages are named after genera, it merely 

 means that the characters of those genera are predominant. 



Retardation plays an important part in the species dis- 

 cussed in this paper, showing itself especially in the septa, 

 causing them to fail to reach the full development of Hof- 

 Ittes, the immediate ancestor of Placenticeras, and preventing 

 individual ontogeny from giving the full ancestral record. 



It is here demonstrated that new elements of the septa, 

 contrary to the accepted belief, are sometimes added by 

 subdivision of primary lobes on the outside, and not neces- 

 sarily always in the part concealed by the involution, thus 

 giving a reasonable explanation of the large number of small 

 lobes found in such genera as Beloceras, Pinacoceras, and 

 Sphenodiscus. The occurrence of lobes of this character 

 does not show that such genera are related, but merely that 

 each is a gerontic form, and that descendants of these are 

 not to be expected in later formations. This does not apply 

 to Arcestes, which has a large number of primary lobes, 

 visible even on the second septum from the protoconch. 



The stages wholly lost out of the ontogeny lie between 

 the nautilian protoconch and the glyphioceran larval stage; 

 this unrecorded part of the development is thought to have 

 corresponded to the time spent in the ^^^. All later stages 

 are recorded in ontogeny with a fair degree of distinctness. 



The protoconch can not be correlated with any nautiloid, 

 but the later stages can be compared with ammonoid genera, 

 the exactness of the correlation becoming less as the stage 

 advances, on account of unequal acceleration of development 

 of ancestral characters, but on the other hand easier, on 

 account of the greater number of characters one has to deal 

 with. 



The earliest larval stage is nautiloid in septa, but ammon- 

 oid in its calcareous protoconch. The middle larval stage 

 is comparable with the Paleozoic group Glyphioceratidse, 

 probably Glyphioceras itself; the last larval stage is analo- 

 gous to JVanvites, a genus characteristic of the earliest 

 Mesozoic. 



