8o NEW YORK STATE MUSEUM 



26.5 mm, an average specimen (living chamber and four camerae) 

 is 35 mm long, its living chamber is 23.5 mm high and. 24.1 mm 

 thick; the sutures are 3 mm apart; the siphuncle is 2.5 mm wide; 

 the two diameters of the section measure 13.5 mm and 15 mm 

 respectively. In another specimen the living chamber is 22.5 mm 

 long and 19.5 mm thick. 



Horizon and locality. Lockport (Guelph) limestone. Niagara 

 Falls. 



Remarks. All American species of Hexameroceras have been 

 obtained in the Niagaran ; H . h e r z e r i Hall and Whitfield in 

 that of Ohio, H. delphicolum and cacabiforme 

 Newell in Indiana. H. microstoma is readily distinguished 

 from the first of these western species by the horizontal course of 

 the hyponomic area of the aperture, which in h e r z e r i slopes 

 rather steeply downward. Also the living chamber of that form is 

 relatively much shorter. Both H. delphicolum and H . 

 cacabiforme are larger types with relatively shorter living 

 chambers. H. delphicolum differs also in having strongly 

 curved suture lines ;in cacabiforme, which is characterized 

 by its very short living chamber, the rest of the conch is unknown. 



The geologic distribution of the genus Hexameroceras is of 

 especial interest. The whole series of the family Trimeroceratidae 

 Hyatt, namely, Mandaloceras, Hemiphragmoceras, Trimeroceras, 

 Tetrameroceras, Pentameroceras, Hexameroceras, Septameroceras 

 and Octameroceras, form a sudden outburst of peculiar aberrant 

 breviconic cephalopods with strongly contracted apertures and 

 highly differentiated brachial and hyponomic areas. They are 

 entirely restricted to the Middle Silurian, the Bohemian types all 

 occurring in stage E and the American species (as those of 

 Hexameroceras named above, Trimeroceras gilberti 

 Kindle and Trimeroceras wabashense Newell, the 

 Septameroceras septoris Hall, Pentameroceras 

 mi rum (Barrande) Newell and a Septameroceras from the 

 Pittsford shale) being all restricted to the Niagaran, with the 

 exception of the specimen from the Pittsford shale which imme- 

 diately follows the Niagaran. They represent, therefore, a typical 

 aberrant group of very rapid development through stages with 

 increase of the number of arms and of corresponding sinuses in 

 the aperture from 2 to 8; and equally rapid extinction. It would 

 be likewise interesting if one could trace the external influences, if 

 there were such, that led to the rapid development of this group. 



