HEREDITY. 419 



whorls broader and having more specialized charac- 

 teristics correlative with closer coiling and the early 

 existence of an impressed zone. It is, therefore, fair 

 to infer that the evidence when accessible will confirm 

 the facts observed in previous periods. "^ 



"The same arguments apply also to the tertiary 

 forms as far as known. 



"The terminal members of the nautiloids are, of 

 course, the existing species. Nautilus pompilius and um- 

 bilicatus have been examined in a considerable number 

 of specimens, and in all of these the impressed zone 

 and correlative helmet-shaped whorl and broad flat- 

 tened dorsal side appears during the cyrtoceran stage. 

 Fig. 119, Nos. 15-16, are outlines of the shell oi pompi- 

 lius during the cyrtoceran stage exhibiting the helmet- 

 shaped whorl, broad dorsum, or inner side, and its 

 impressed zone, iz. Thus, when the whorls touch, as 

 in all the nautilian shells of the Carboniferous, Jura, 

 and Cretaceous, in which the same acceleration of de- 

 velopment also occurs, the whorl is already prepared 

 to become involute and to mould itself more readily 

 and rapidly over the surfaces of the apex and the side 

 of the succeeding whorls. In other words, heredity 

 has begun the work before the whorls touch, and be- 

 fore the deepening and enlargement of the impressed 

 zone through the pressure of close coiling is begun. 

 There are quite a number of characteristics of the spe- 

 cies of existing Nautili which lead to the inference 

 that they are survivors of Jurassic and generalized 

 Cretaceous and Cenozoic forms ; the size of the um- 

 bilical perforations, the smoothness of the shells, the 

 simplicity of the sutures, and so on. These facts are 



IThis inference has been fully sustained by subsequent investigations.— 

 A. Hyatt. 



