Dr. BiGSBY on the Geography and Geology of Lake Huron. 197 



Their transverse section is more frequently oval than circular; but both 

 forms occur. 



The column is divided into chambers from the top to the bottom by septa, 

 which are usually concave toward the larger end ; but in three specimens, 

 which I am inclined to consider as orthocerae, PI. XXVI. fig. 3, 6, and 11, 

 found by my friend David Thompson Esq. in the island above referred to, near 

 Thessalon, the septa are convex upwards. 



The interval between the septa, or the depth of the chambers, is for the most 

 part either constant in the same individual, or diminishes toward the lower 

 part of the column ; but in two specimens (see PI. XXVI. fig. 3 and 9), the 

 former from the island near Thessalon, the latter from St. Joseph's, the depth 

 of the chambers varies irregularly in different parts of the same column. In 

 different individuals of the same species, the greater the column the greater is 

 the depth of the chambers ; but in different individuals of different species, the 

 depth of chambers bears a variable proportion to the size of the column. In 

 one of the specimens above referred to, fig. 11, the column of which is five 

 inches broad, and diminishes rapidly, the chambers are very shallow and nu- 

 merous. Figures 2 and 3 are in the most usual proportion. 



In some species the siphuncle is near the centre, in others near the circum- 

 ference ; and when eccentric, in those species that are oval, it is usually placed 

 in the shorter axis of the section. In the same species the diameter of the 

 siphuncle is in proportion to that of the column, but in different species the 

 proportion of the two diameters is very variable. In PI. XXVI. fig. 8 and 1, 

 the siphuncle is larger in comparison of the column, and in PI. XXVI. fig. 10 

 is smaller than is usual. 



The siphuncle, longitudinally considered, is in some instances a continuous 

 tube, regularly tapering from the top to the bottom ; in others it is contracted 

 in its passage through the septa, and enlarged in the intervals between them 

 (see PI. XXVI. fig. 2), as in the O. cordiformis of Sowerby. An orthoceratite 

 from Malbay has the same character. 



In figure 7 there is the appearance of a small interior tube within the si- 

 phuncle. 



In figures 1 and 8, where the siphuncle is large, there are peculiar mark- 

 ings at the upper end, which doubtless are connected with the organization 

 of that part. I have seen other instances of this in larger specimens. 



In fig. 6 the siphuncle is marked along the side by a series of broad arrow- 

 heads, pointed upvvard.s, and diminishing as they descend. They are merely 

 superficial markings, corresponding in number with the chambers. This spe- 

 cimen is 5 inches broad at the top, and 13 inches long. 



