HEREDITY. 417 



Thus these characters, although purely ctetic in origin, 

 were repeated before the usual conditions recurred in 

 the ontogeny of this species which had obviously and 

 repeatedly produced them in the nautilian forms of the 

 earlier Paleozoic and the more generalized 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 Fig. iig. No. 

 II, and a peculiar aperture. 



"The Triassic period is unimportant in this con- 

 nection since it has but few nautilian species that are 

 deeply involute and also sufficiently well known to 

 throw any light upon this problem. All of the true 

 orthoceran, cyrtoceran, and gyroceran forms diminish 

 in the Carboniferous and disappear with the Trias. 



"The Jura contains a considerable number of nau- 

 tilian shells of different genera of which the cyrtoceran 

 stages are sufficiently well known. Cenoceros aratum, 

 of which several specimens have been studied, shows 

 the impressed zone and correlative characters in this 

 stage ; C. lineatum is the same ; C. clausum, same ; 

 C. intermedium, same. Fig. 119, No. 14, shows the 

 cyrtoceran stage in a shell of C. clausum, with a well 

 developed impressed zone, i. z. Endolobus is a char- 

 acteristic 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 instruc- 

 tive to note that this species has the impressed zone, 

 according to D'Orbigny's figure, 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 correla- 

 tive characteristics are inherited in most species of the 



