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(1.) Plectoceras Jason, Billings. (Sp.) 



Plate 36, figs. 1 and 2 . 



Jfautilus Jason, Billings 1859. Can. Nat. and Geol., vol. IV, p. 464. 



Plectoceras Jason, Hyatt 1883. Proc. Boston Soc. Nat. Hist., vol. 



XXII, p. 268 ; and (1894) Proc. Amer. 



Philos. Soc, vol. XXXII, p. 498. 



Original description of Nautilus Jason. " Discoid, planorbiform, all 

 the whorls exposed in the umbilicus. Section of shell broad oval, the 

 ventral and dorsal sides being depressed convex, the other two sides 

 rounded, the two diameters being to each other as fifteen to seventeen. 

 The tube increases in diameter at such a rate as to give to the coil a 

 diameter of three inches and a half on the completion of the second whorl, 

 at which point the septa are two lines distant from each other in their 

 centres, two and a half lines in the middle of the ventral side, and a little 

 less than one and a half lines on the dorsal or inner side of the whorl. 

 They become gradually more approximate as they approach the apex, so 

 that where the tube is half an inch in diameter they are one line distant 

 in their centres. They are only moderately convex, and their edges cross 

 the ventral aspect in a straight line, but on the sides with a gentle curve 

 towards the apex of the shell. 



"The surface exhibits a series of rounded ridges which, starting from 

 the umbilicus, curve backward, and make a deep rounded undulation 

 toward the apex on the ventral aspect. The distance of the ridges from 

 each other along the median ventral line is about five lines, and the inter- 

 vening spaces are shallow and concave. The surface is also marked with 

 obscure fine strire, and smaller ridges all parallel with the larger. 



" The siphuncle is from one and a half to two lines in diameter, cylin- 

 drical, not inflated, and distant about two lines from the outer margin. 



" On comparing this species with the figures of L. undatus (Emmons) 

 given in the Palaeontology of New York, vol. 1, plate 13, we find that our 

 best preserved specimen is exactly the size of figure 1 ; that the ventral 

 aspect is not angular at the sides, nor do the ridges pass straight across, as 

 shewn in fig. 1 h ; and that in fig. 3 the septa are more than three lines 

 distant in their centres instead of two lines, as they are in our specimens. 

 The specimen represented on plate 13 his has the septa three and a half 

 lines nearly distant about the completion of the second whorl, while those 

 next the chamber of occupation are more approximate, as they usually are 

 in all the Nautilidse. Our species therefore, although closely allied, is a 

 distinct species from L. undatus. 



" Locality and Formation.— Mingan Islands ; Chazy limestone." 



The foregoing description was evidently based upon three fine and 

 remarkably well preserved specimens from Clear Water Point, in the 



