1883.] 297 [Hyatt. 



The presence of a pair of large tubercles on the chambers of hab- 

 itation in some of the species unites them with such forms as 

 Asypm. (Naut.) bifrons sp. DeKon. Calc. Carb. pi. 16. Even the 

 contracted chamber of this species and of Asypm. (Naut.) con- 

 spicuum ibid, pi. 19, does not enable us to separate these 

 species. 1 



JVautilidae. 



The species of this family have the typical nautilian whorls, 

 ridged in the young, but smooth usually in adults. The sutures, 

 though lobed on the venter in one genus, have generally broad 

 saddles and sutures like those of the recent Nautilus. In several 

 series we traced the appearance of the annular lobe in the sut- 

 ures, and an internal septal depression, which we have called the 

 cone. The sutural lobe appears in the Devonian, and is inherited 

 in the Triassic genus, Cenoceras, with the cone better developed, 

 and becoming separated from the sutural lobe. In different gen- 

 era springing from this common type there is a tendency towards 

 concentration in the young, the cones being confined to the 

 earlier stages. Thus in the Jura the cones are still occasionally 

 found in adults, but in the Cretaceous probably very rarely, if at 

 all, and in the Tertiary and present no case of this kind was ob- 

 served, though they are characteristic of the young. The lobes 

 in the sutures are not exclusively confined to the larval and earlier 

 adolescent stages of growth of recent species, as are the cones, 

 but may be present in adults of the existing Nautilus. In con- 

 sequence, however, of their separation from the cones, they be- 

 come easily obliterated in fossils, and are apt to escape observa- 

 tion. 2 



1 Asympt. (Naut.) dorsale is usually considered the type of D'Orbigny's genus 

 Cryptoceras, Prod, de Pal. p. 114, but the species first mentioned by that author on 

 p. 58, Tem. subtuberculatus, should be considered as the type. 



2 This is also the history of the same part in other series besides this family and is 

 applicable to all the families of Nautiloids in which the annular lobes appear. The 

 annular lobe of the suture, and the accompanying depression of the septa are almost 

 invariably united in the Paleozoic forms, and the appearance of the cone is very rare. 

 The separation of the two is, however, the rule in the Mesozoic, as detailed above in 

 the Nautilidae. In the Ammonoidea, on the contrary, the lobes and cones appear 

 though very rarely, in Silurian species, and are fully developed m the Devonian Gon- 

 iatites. 



