118 



INFERIOR OOLITE AMMONITES. 



convex, ornamented with sigmoidal radii, of which the outer arc is longer than the 

 inner — the latter, too, being inconspicuous. The ribs gradually increase in size until 

 they become most pronounced on the edge of the ventral and lateral areas ; then, 

 slightly decreasing, they sweep forwards and die away, at the edge of the ventral 

 furrows, to inconspicuous lines of growth. Ventral area slightly subconvex, but 

 interrupted by a small solid carina and two furrows, one on each side. Inner 

 margin convex. Inclusion variable, amount of exposure of preceding whorl being 

 from one half to almost nothing in different specimens. Umbilicus also variable, 

 in some specimens being small and deep, and in others open and flat. Termination 

 plain, sigmoidal, with a rounded lateral, and a pointed ventral, process. The 

 length of the body-chamber is O60 of a whorl in fig. 1. 



This species is very variable in shape, as an examination of the following Table 

 will show. 



Diameter 



100 . 



.. 100 . 



.. 100 . 



.. 100 . 



.. 100 . 



.. 100 . 



.. 100 



.. 100 



Height of last whorl . 



50 . 



.. 50 . 



.. 44 . 



.. 54 . 



.. 48 . 



.. 50 . 



.. 42 



.. 50 



Breadth of last whorl . 



67 . 



.. 57 . 



.. 39 . 



.. 39 . 



.. 38 . 



.. 46 . 



.. 31 



.. 32 



Umbilicus . 



20 . 



. 20 . 



. 25 . 



.. 14 . 



. 23 . 



.. 24 . 



.. 37 



.. 21 



Inclusion . 



25 . 



.. 23 . 



.. 17 . 



.. 21 . 



.. 23 . 



.. 17 . 



.. 10 



.. 21 



Some individuals are very thick, with the breadth of the whorls considerably 

 more than the height, and with a very small umbilicus ; others have a very small 

 umbilicus, but the height of the whorls greater than the breadth ; others again 

 have the umbilicus open, even considerably so, and the height of the whorls 

 less than the breadth. At first sight there appear to be three chief varieties of 

 this species, namely, (1) thick with closed umbilicus, (2) medium with open 

 umbilicus, (3) thin with very open umbilicus. An examination, however, of a 

 large number of specimens shows that this will not hold good ; for instance, the 

 specimen whose proportionate dimensions I have given under No. 4 shows the 

 smallest umbilicus, but with very narrow whorls. 



Dr. Haug appears 1 to be of the opinion that the open-centred, flat forms are 

 the young state of the thick, narrow-centred specimens ; but the specimens 

 which I have had figured will clearly indicate that this is not the case. For 

 instance, a young specimen (Plate XXII, figs. 19, 20) of the very thick form 

 shows that the shape is practically the same in youth as in adolescence (Plate 

 XXII, figs. 13, 14); and this is the case with all the different forms of the 

 species. 



The following are some of the variations which may be noted : 



a. (Plate XXII, figs. 1, 2, 7, 8.) Medium form. 



1 Op. dt., p. 639. 



