HEREDITY. 413 



rocks of the Quebec group. There is, however, this 

 quaHfication : the fifth class of forms, or the involute 

 nautilian, are relatively rare and become more abun- 

 dant in successive periods. The young of nautilian 

 shells of the earlier periods are also apt to be less closely 

 coiled, or, in other words, remain open and similar to 

 cyrtoceras for a longer time during their growth. This 

 is shown by the large size of the central hole, or um- 

 bilical perforation, left in the center of full-grown 

 shells. This perforation is much larger, as a rule, in 

 Paleozoic than in the Mesozoic forms. 



" In each period the genetic series or groups of nau- 

 tilian forms have peculiarities of structure in the su- 

 tures, ornaments, apertures, etc., by which they can 

 be separated from each other, and these peculiarities 

 are the same as those possessed by gyroceran, cyrto- 

 ceran, and often orthoceran shells which occurred often 

 earlier in time, so that one can trace each group of 

 nautilian shells back to its ancestors through the par- 

 allel stages of evolution above described. The groups, 

 in other words, are parallel in their morphogenesis, 

 like two individuals of the same parents in their de- 

 velopment from youth to old age. 



'*As a general rule the impressed zone originates, 

 as described above, after the whorls come in contact, 

 rarely before this time in the growth of any individ- 

 uals. Barrandeoceras is one of the most involute shells 

 known in the Silurian, and Fig. iig, No. 6, gives a 

 true sketch of this species ; No. 7, shows a section of 

 a full-grown shell with a decided impressed zone, and 

 No. 8 is the young. This last is a purely cyrtoceran 

 form with a compressed elliptical section like that of 

 No. 7, but no impressed zone, the inner side being 

 rounded like the diagram of Cyrtoceras, No. 2. The 



