MUSEUM OF COMPARATIVE ZOOLOGY. 79 



species are figured by D'Orbigny, and these are decidedly in advance 

 of the circular and very slowly increasing whorls of the young of the 

 Carboniferous and Silurian Nautili, and the umbilical perforations are 

 much smaller. Barrande says of the umbilical perforation, " Au centre 

 de la spire, il existe une vide ou perforation, qui est remarquable par sa 

 Constance, et son etendue dans les Nautiles paleozoiques. Nous re- 

 trouvons cette perforation, quoique tres-reduite, dans les especes fossiles 

 des epoques posterieures et meme dans les Nautiles qui vivent dans 

 nos mers." The Jurassic shells belong almost wholly to the Striati, 

 a group with longitudinal ridges or plications on the whorls. 



The young of Nautilus lineatus of the Jura presents an umbilical 

 perforation quite as small as that of the modern Nautilus. The whorl, 

 however, does not make the graceful curve of Nautilus Pompilius, but 

 bends inward more abruptly, and instead of touching the apex of the 

 whorl first, it strikes the dorsal ridge of the area of the cicatrix, fitting 

 itself to its flattened surface.* The result is an irregularity in the 

 curvature of the dorsum at this point, which appears to be abnormal, 

 but is probably characteristic at le?st of the species, since I found it 

 in two different specimens. 



This fact, however, as well as the figures of the young given by 

 various authors, shows that in the" Jura the whorling probably becomes, 

 in several species, as close, and perhaps closer than in the modern Nau- 

 tilus ; certainly, in all those forms which, like Nautilus lineatus, are 

 very involute in the adult. 



The Radiati of the Cretaceous are, as a whole, more deeply involute 

 than the Striati of the Jura, though not differing as respects the size 

 of the umbilical perforation. The earlier age at which the involution 

 begins is particularly noticeable, and the consequent prevalence of 

 forms which increase in size more quickly than the majority of Jurassic 

 species may be assumed with confidence, though the material at my 

 command does not enable me to substantiate this statement by actual 

 observation made upon the uncovered young. 



It is founded, however, upon the fact which appears to be universal 

 among Ammonoids and Nautiloids, that the earlier a species begins to 

 become involute, the quicker must be its increase in size. Involution, 

 indeed, is only one method of expressing the expansion of the shell in- 

 wardly by the growth of the sides over the umbilical area, and it is 



* Plate IV, Fig. 10. 



