BEEKMANTOWN AND CHAZY FORMATIONS OF CHAMPLAIN BASIN 483 



and sutures similar, but with ventral saddles. The whorls quadrate, 

 the abdomen narrower than the dorsum and the sides convergent out- 

 wards. The siphons are ventral and holochoanoidal. The young 

 are precisely similar in form, smoothness of the shell and striae of 

 .growth, and in sutures to the straight sutured form of Trocholites. 

 Type : P 1 e c t . ( Naut.) Jason sp. Bill. 



To this first diagnosis it has later [Hyatt 1894] been added that 

 the mode of coiling may be quite close and regular, with perhaps a 

 slight impressed zone or flattened dorsum, or the coiling may be 

 open, and sometimes very irregular; that the umbilical perforation 

 is large ; the impressed zone absent until the whorls come into con- 

 tact and invariably absent in gerontic whorls. 



The genus Plectoceras has then further been made the type of the 

 family Plectoceratidae. A comparison of the latter with the family 

 Tarphyceratidae will show that the essential difference lies in the 

 presence of " annular costae from the neanic stage until late in life ". 

 Since, however, costation becomes already quite strong among the 

 Tarphyceratidae, as in T . p e r k i n s i — a form which on account 

 of the prominence of the costae was separated by Whitfield from T . 

 champlainense — it is quite apparent that a sharp separa- 

 tion between the forms of the two families will frequently be quite 

 difficult or even impossible. Indeed Whiteaves has lately [1903, 

 p. 121] suggested that the frequently cited Black river form, 

 Nautilus undatus Hall, should be referred to Plectoceras 

 rather than to Eurystomites as Hyatt had done. We agree with 

 Whiteaves that the adults of N . undatus do not show any dif- 

 ferences of generic rank from Plectoceras. Moreover, it is conceded 

 "by Hyatt himself that also in N . undatus the costae appear al- 

 ready in the neanic substage. 



According to the phylogenetic principles followed by Hyatt in his 

 classification we would have to see in Plectoceras a group of forms 

 advanced beyond the Tarphyceratidae, an assumption which is in 

 accordance with the geologic range of the genus, for while the Tar- 

 phyceratidae are prevailingly of Beekmantown age, Plectoceras does 

 not begin until Chazy time with the species here described, ranges 

 through the Black river stage with P . h a 1 1 i and is still recorded 

 from the Niagaran. From the species known thus far it would 

 appear that the genus is restricted to the American basin. 



