Hyatt.] 402 [Nov. 16, 



macrosiphonulate probably even in the adult stage, since it in- 

 creases in diameter throughout life. Piloceras had a huge siphon 

 hardly at all contracted in the adults of some species, but consid- 

 erably lessened in diameter during the same stage in others. En- 

 doceras had also a large siphon always more or less contracted 

 in the silphologic or later stages. The uncontracted macrosiphon- 

 ula occupied in this genus a number of air chambers varying ac- 

 cording to the species, from a few to six or more. This was 

 evidently due to the earlier inheritance or concentration of the ten- 

 dency to decrease the diameter of the siphon first manifested in 

 the adults of Piloceras. Sannionites was a genus in which the 

 siphon was smaller than in Endoceras, and probably, though this 

 is not yet ascertained, inherited the tendency to microsiphonu- 

 lation at the first septum at an earlier age than in Endoceras. None 

 of these forms, however, attained a true microsiphon, since even 

 Sannionites had the siphon filled by enclocones and in the centre an 

 endosiphon. These organs entirely disappeared in true microsi- 

 phonulate forms and, in fact, could have existed only within a 

 large siphon. 



Nevertheless this tendency to decrease the size of the siphon re- 

 sulted in the formation of a definite constriction. This constric- 

 tion was inherited at earlier and earlier stages after its origin in 

 the siphon of Piloceras, until it became constant perhaps in Sanni- 

 onites and certainly in the Orthoceratidoe. The constriction 

 marked the line between the larger and smaller siphon in the 

 macrosiphonulate forms, and, in becoming constant through con- 

 centration, it became invariably fixed behind the first septum, be- 

 tween the csecosiphonula, and the smaller siphon. This smaller 

 siphon though still a macrosiphon in structure as explained above, 

 even in Sannionites, was undoubtedly transitional to the true mi- 

 crosiphons of the Orthoceratidse. 



The csecosiphonula was in all Orthoceratites, which are other- 

 wise similar to Endoceras, confined by concentration of develop- 

 ment to the first air chamber, and a true microsiphonula appeared 

 at an early stage as an open narrow tube. This was similar to the 

 siphon of the vast majority of all succeeding forms of both Nau- 

 tiloids and Ammonoids. According to the classification here 

 advocated, the stages preceding the microsiphonula, viz. : the 

 asiphonula, cseeosiphonula and macrosiphonula, became silpho- 

 logic stages in all the groups of Cephalopoda descending from the 



