Fossils of the Chazy Limestone. 465 



sal 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 ihe 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 dor- 

 sal 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 cen- 

 tres. 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 towards the apex on the ventral aspect. The distance 

 of the rido-es from each other alono- the median ventral line is 

 about five lines, and the intervening spaces are shallow and con- 

 cave. The surface is also marked with obscure fine strire, and 

 smaller ridges all parallel witli the larger. 



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

 cylindrical, not inflated, and distant about two lines from the 

 outer margin. 



On comparing this species with the figures of L. undatus (Em- 

 mons), 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. 16/ and that 

 in fig. 3 the septa are more than th"ee lines distant in their cen- 

 tres instead of two lines, as they are in our specimens. The spe- 

 cimen represented on plate 13 bis has the septa three and a half 

 lines nearly distant about the completion of the second whorh 

 while those next the chamber of occupation are more approxi- 

 mate, as they usually are in all the Nautilidre. Our species there- 

 fore, although closely allied, is a distinct species from L. undatus. 



Locality and Formation. — Mingan Islands ; Chazy limestone. 



Nautilus tyrans, N. s. 



Description. — Discoid planorbiform, all the whorls exposed in 

 the umbilicus. Shell increasing in size, so that at the completion 



Canadian Nat. 5 Vol, IV. No 6. 



