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of Trigonoceras are closely parallel. If the adults were not known 

 they would be referred necessarily to that genus. 



The group is of importance in the history of the impressed zone 

 since it shows in its most specialized and highly involute members 

 that a contact furrow may appear even in a form of whorl that has 

 naturally a gibbous dorsum and concave abdomen. 



Fig. 2, PI. xii, of Diorugoceras (Nant.) planidorsatum (sp. Port- 

 lock), Hyatt shows the peculiar character of the contact furrow in 

 these forms when it occurs. 



It is probable that the early neanic stage has a gibbous dorsum 

 fitting into the hollow abdomen and that the involution is acquired 

 rapidly in the later substages of the neanic stage, but not having 

 seen specimens of the young I cannot state this as a fact. 



Triboloceratidcz. 



The figures of Thoracoceras puzosianum of PI. ix show a shell 

 which in form is a slightly depressed oval and both in this respect 

 and in the fluted ornamentation approximates to the nepionic stage 

 of Thoracoceras canalicidatum and other subspinous forms of the 

 same genus. This last species has also a similar form, and by com- 

 paring this with the young of the loosely coiled, gyroceran forms 

 on the same plate, Figs. 14 and 15 of Triboloceras, it will be seen 

 how closely they resemble them. Triboloceras in turn grades into 

 the nautilian form of the same family, Vestinautilus Konincki, 

 Figs. 5-13. The figures show the development of this form 

 through a nepionic stage which is at first similar to T. puzosianum, 

 then becomes similar in ornamentation to T. caualiculatum and 

 then passing into the neanic stage these primitive characters are 

 replaced by the peculiar acquired ornamentation and whorls having 

 the hollow, central, ventral and lateral ventral zones of this family 

 and smooth, gibbous, umbilical zones with broad, fluted, lateral 

 faces. The subspinous ornamentation persists in this form on the 

 ridges throughout the ephebic stage. In the gerontic stage these 

 progressive characters disappear and with them the fluted faces and 

 zones also tend to extinction and in the paragerontic stage do actu- 

 ally give way to a rounded form without salient angles. This last 

 is not figured, but the tendencies towards extinction of the orna- 

 ments, etc., may be seen in the anagerontic substage delineated in 

 Figs. 5 and 6. 



