484 



NEW YORK STATE MUSEUM 



Plectoceras jason Billings (sp.) 



Plates 29-31 



Nautilus jason Billings. Can. Nat. and Geol. 1859. 4:464 

 Plectoceras jason Hyatt. Bost. Soc. Nat. Hist. Proc. 1884. 22:268 

 Plectoceras jason Hyatt. Am. Phil. Soc. Proc. 1894. 32:499 

 Plectoceras j aso n Whiteaves. The Ottawa Naturalist. 1903. 17:120 



The Chazy bed B^^ of the Valconr section has furnished five speci- 

 mens of a coarsely costate, middle sized nautiloid which we have 

 foimd to be identical with a form described by Billings in 1859 froin 

 the Chazy limestone of the Mingan islands in Canada as Nautilus 

 jason and which later on was made the type of the genus Plecto- 

 ceras by Hyatt. 



Description. Conch a nautilicone which as a rule is quite 

 closely coiled ; consists of three to four volutions, and attains in a 

 specimen of not quite three volutions a diameter of 125 mm. Rate 

 of growth moderate, the dorso ventral diameter attaining its double 

 length within one volution. Sections of early volutions subcircular, 

 the two diameters about equal, the abdomen slightly flattened, the 

 dorsum slightly impressed and the sides rounded; in the ephebic 

 stage and still more so in the gerontic stage, the section is sub- 

 quadratic, the abdomen and the. dorsum being broad and flat, the 

 latter more so than the former and the sides slightly convergent 

 outward. 



Figs. 42, 4:1 Plectoceras jason Bill, (sp.) 



Fig. 42 Transverse section throug;h the living chamber of the speci- 

 men reproduced on pi. 29, x J^ ; fig. 43 view of last septum, at base 

 of living chamber reproduced on pi. 30, somewhat deformed by- 

 lateral pressure, natural size 



Living chamber less than one half volution, in gerontic species 

 slightly uncoiled. Aperture with very deep hyponomic sinus, the 

 ventral margin in the ephebic stage receding behind the dorsal mar- 



