AEGOCERAS CHARMASSEI. 325 



metres, it is moderately compressed, the whorls are nearly one half involute, and their 

 convex sides near the umbilical margin are covered with twenty well-marked ribs, which 

 bifurcate near the middle of their height, and enlarge as they advance toward 

 the margin, where they terminate in thick blunt knobs. The siphonal area is narrow and 

 smooth, sometimes concave in the middle, and ribbed at the sides by the enlarged 

 terminations of the costse. The spire is composed of several moderately thick whorls, 

 which are convex on the sides and bevelled away near the margin. The aperture is 

 elongated, narrow, and sagittate. 



In middle age, which is fairly well represented by the fine specimen from Lyme 

 Regis, figured in PL XX, we observe all the characters which distinguished its earlier 

 life very well developed in this specimen. The number of primary ribs on the last 

 whorl at the umbilical margin now amount to thirty, and the secondary ribs arising 

 from the bifurcation of the primaries amount to sixty. The thickening of the secondary 

 ribs as they approach the margin, and their final termination in a rounded knob -like 

 eminence, are very characteristic of this species. 



In old age this Ammonite attains a considerable magnitude. I have one specimen 

 which measures 320 millimetres in diameter, and yet wants a considerable portion of the 

 body-chamber. The shell is much compressed, and the relative proportions of its different 

 parts are much changed. The ribs have gradually become much smaller and less promi- 

 nent until they have finally disappeared, and left the sides of the fossil quite smooth. 

 The umbilicus is proportionately much less in width to the total diameter of the shell ; 

 the last whorl is much higher, and not nearly so wide in proportion ; the siphonal 

 area, from the bevelling away of the deep sides, has become very narrow, and its promi- 

 nent centre forms quite a carina— in fact, had the student not watched and noted the 

 changes that take place in the growth of this Ammonite, he could not believe, on 

 comparing the young with the old shell, that he was only examining the same species 

 passing through its various morphological phases of evolution. 



The lobe-line is extremely tortuous in this species, as shown in PI, XX, fig. 1, and 

 three large and three small oblique auxiliary lobes, with three large and three small 

 oblique saddles, are shown on the sides of the whorls. The windings of the lobe-lines 

 from adjoining septa are, however, so closely approximated that it is extremely difficult 

 to trace the one line continuously from the siphonal to the columellar lobe. The siphonal 

 lobe is as wide, but not so long, as the principal lateral, and has on each side three large 

 branches with multi-digitate terminations ; the siphonal saddle is as wide as the principal 

 lateral, and is deeply divided into three irregular folioles with their sides much festooned ; 

 the principal lateral is large and very conspicuous, it has a pyramidal form, with three 

 long branches on its outer, and four on its inner side, and a long terminal multi- 

 digitate process ; the lateral saddle resembles the siphonal in size and in the complexity 

 of its ramifications ; the lower lateral lobe is very much smaller than the principal, which 

 it resembles much in the style of its digitations ; the auxiliary lobes, three or four in 



