﻿and, although always present, they are only occasionally developed 

 into decided costations even in full-grown specimens. 



The shell grows quite large, but so far as I have seen none have 

 any part of the last whorl free. The largest shells often have com- 

 pressed whorls, with abdomens much narrower and flat, and sides 

 much flatter than in the earlier stages. 



Planctoceras. 



This group was first described by Remele under the name of 

 ^Egoceras and subsequently under that of Tragoceras, but both of 

 these being preoccupied, Schroder proposed that of Planctoceras.* 



Schroder considers it to be a subgenus, and that the only dis- 

 tinction between this and Estonioceras lies in the fact that it was 

 probably not close coiled at any stage. 



As Estonioceras is here limited, however, the sutures are differ- 

 ent and have ventral and dorsal saddles with lateral lobes, as in 

 Falcilituites. The young and all stages so far as seen have also 

 compressed elliptical instead of depressed whorls. That is to say, 

 they are probably never digonal, and do not resemble those of 

 Estonioceras at any stage, unless in the very earliest or nepionic 

 stage which is not known. The whorls, as shown by Schroder in 

 his fine figures, have the dorsum and venter somewhat depressed 

 and very nearly equal and distinct from the side in the young. In 

 other words, there is a faint tendency to form a quadragonal whorl. 

 Later, probably in the ephebic stage, the dorsum may exceed the 

 venter in breadth, and in the gerontic stage the whorl becomes al- 

 most circular. 



The lines of growth are similar to those of Falcilituites, i. e., 

 they have broad ventral sinuses and a broad latero-dorsal crests. 



The volutions are attenuated and the living chambers very long. 



The siphuncle is small and about twice its own diameter removed 

 from the venter, or, in my nomenclature, is extracentroventran in 

 the neanic and ephebic stages as measured on Schroder's figures. 



The only species referred to this genus in Europe is the Planctoceras 

 (Orthoceratites) falca turn, sp. Schlot., which, judging from the fig- 

 ures of Dewitz, Schroder, Quenstedt and Eichwald, probably in- 

 cludes several quite distinct species. Planctoceras Quenstedt (Lit., 

 falcatus, Quenst.), for example, has distinct sutures and outline 



* Dames et Kayser, Pal. Abh., v, lift. 4, 1891, p. 41. 



