182 
fully im sitd in fig. 32. The length and capacity of the body- 
chamber varies considerably in the different genera, which is 
found to have proportionate dimensions to that of the outer 
whorl; this appears to be constant in several of the groups on 
which careful observations have been made; the difficulty 
experienced in obtaining specimens in which the body-chamber 
is preserved is, however, a sufficient reason why more has not 
hitherto been done in this line of inquiry. 
In Arietites (fig. 35) it will be observed that the septa are 
convex towards the aperture and concave towards the spire, 
whilst in Nautilus we have seen that the opposite condition 
prevails. The body chamber in Arietites is likewise very long, 
extending from one whorl to one whorl and a half in length. 
Fie. 35.—Section of Arietites obtusus, Sow. 
Showing the size of the dwelling-chamber and outward convexity of the septa. 
In Aegoceras the length of the body-chamber varies from 
two thirds of a whorl to an entire whorl. 
In Amaltheus the body-chamber is short, and varies from one 
half to two thirds of a whorl. ’ 
In Harpoceras it is about two thirds of a whorl. 
In Stephanoceras it is from one whorl to one whorl and a 
quarter in length. In Perisphinctes from two thirds to a whole 
whorl, and in Cosmosceras it is about half a whorl in length. In 
Phylloceras it is short and wide, and in Lytoceras it is round and 
two thirds of a whorl long. 
Fourthly.—The shell of the AmmonrrT1pz is a cone, which is 
more or less rolled up upon the same plane or in a spiral, and 
