228 THE LIAS AMMONITES. 



Fig. 132.— Section of Arietites qhtusus. 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 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 and a quarter in length. In 

 Perisphindes 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 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, like Arietites (fig. 108), have a wide umbilicus, with 

 their inner whorls largely exposed ; in others, as Amaltheus and Harpoceras, the whorls 

 are much covered by the preceding whorl ; in some species of Phylloceras they 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. E. 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 should, but 

 from the circumference of the embryo; and it has, therefore, been proposed by 



1 ' The Yorkshire Lias,' p. 262. 



