228 
THE LIAS AMMONITES. 
Pig. 132. — Section of Arictites ohtusus, Sow. Showing the size of the body-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 Amalilieus 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 and a quarter in length. In 
Perisphincies 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. >r ' 
Fourthly. — The shell of the Ammonitid^ is a cone, which is more or less rolled up 
upon the same plane or in a spiral ; and the various turns of the shell or the whorls, as 
they are called, in general cover to a greater or less extent the preceding whorl ; this 
is called the amount of involution of the whorls, a feature in the diagnosis of the shell 
which requires consideration when taken in connection with the other features I have 
described, as the extent of the involution is found to be generally the same in the different 
species of different groups. Some shells, for example, as those of Lytoceras (fig. 123), 
are only slightly involute ; and others, hke Arietites (fig. 108), have a wide umbilicus, with 
their inner whorls largely exposed ; in others, as Amaltheus and HarjJoceras, the whorls 
are much covered by the preceding whorl ; in some species of Phyllocerasiliey are entirely 
enveloped ; and in others the umbilicus is completely closed. This character, the amount 
of whorl involution, appears to depend on the angle at which the shell bends round in the 
process of growth, and as it appears to be a very constant feature, is of value in forming 
a diagnosis of generic characters. On this subject the Rev. J. F, Blake observes,^ " If we 
take any fixed point in relation to the shell — say a point in its surface or in the centre of 
its apertures, that point will describe a curve with the growth of the shell ; and if this 
curve be projected on a plane it nearly forms the well-known ' equiangular spiral 
not exactly, however, because the growth does not begin from a point as it sliould, but 
from the circumference of the embryo; and it has, therefore, been proposed by 
1 ' The Yorkshire Lias,' p. 262. 
