1893.] Phylogeny of an Acquired Characteristic. 873 
ized genetic series of the Carboniferous. That this species, Col. 
globatum, is a highly specialized species is shown by other 
characteristics, especially the early inheritance of a furrowed 
abdomen, shown at v in Pl. XVIII, fig. 11, and a peculiar 
aperture. 
The Triassic period is unimportant in this connection since 
it has but few nautilian species that are deeply involute and 
also sufficiently well known to throw any light upon this prob- 
lem. All of the true orthoceran, cyrtoceran and gyroceran 
forms diminish in the Carbóniferous and disappear with the 
Trias. 
The Jura contains a considerable number of nautilian shells 
of different genera of which the cyrtoceran stages are suffici- 
ently well known. Cenoceros aratum, of which several speci- 
mens have been studied, shows the impressed zone and corre- 
lative characters in this stage; C. lineatum is the same; C. 
clausum, same; C. intermedium, same. PI. XVIII, fig. 14, shows 
the cyrtoceran stage in ashell of C. , with a well devel- 
oped impressed zone, i. z.  Endolobus is a characteristic 
paleozoic type and there is a single survivor of this 
series in the Jura, End. (Naut.) excavatum sp. D’Orb. It is, 
therefore, very interesting and instructive to note that this 
species has the impressed zone, according to D’Orbigny’s fig- 
ure, during the cyrtoceran stage. This species has a large 
umbilical perforation and is slower in coiling up than other 
Jurassic species. The evidence that the impressed zone and 
its correlative characteristics are inherited in most species of 
the Jura before the habit of close coiling could have acted 
upon the whorls so as to produce this modification is, therefore 
very general and convincing. 
The leading characteristic of parallelism in all genetic 
series of Nautiloids is, as may be inferred from the facts cited. 
a tendency toward closer coiling and greater involution in the 
more specialized forms of each separate series and a correla- 
tive increase in the profundity of the impressed zone. When 
the impressed zone becomes inheritable in some closely coiled 
and involute specialized shells of the Carboniferous and in 
similar shells in about all of the genetic series of the Jura 
