110 INFERIOR OOLITE AMMONITES. 



and crenulated on its upper edge by the ends of the knobs or ribs. Inclusion 

 about one-third of the preceding whorl ; the outer part of the umbilicus is rather 

 flat, the inner becoming steeper. The inner edge of the body-chamber of this 

 species shows a slight tendency to recede from the regular line of coil, and the 

 body-chamber itself does not continue to increase in breadth at the same propor- 

 tionate rate that other parts of the shell exhibit ; consequently the outer portion 

 of the umbilicus is wider and flatter, but half a whorl back it is smaller and 

 deeper with steep sides. 



This species has a great resemblance to Lillia comensis, von Buch ; but it is 

 altogether squarer in form, is thicker, has a flatter ventral area with more marked 

 furrows, a smaller umbilicus, and a shorter aperture. 



Judging from the matrix, the horizon of this species is the Humpliriesianum- 

 zone. When it is considered that all the other species of Lillia occur either in the 

 Commune- or the Jurense-zone, and that none are known between this latter 

 and the Humphriesianum-zone, it is very singular to find a species, with such an 

 evidently close resemblance to those earlier forms, at an horizon so much higher. 



There appears to be a very remarkable convergence between Lillia and certain 

 degenerate species of Sonninia, 1 even if this species should happen to be located in 

 the wrong genus. This latter idea, however, I am not at all prepared to admit. 

 This specimen is well preserved, and shows no signs of rudimentary spines in its 

 inner whorls; 2 while the knobs on its inner area are somewhat different to the 

 prominences seen on the specimen of Sonninia (Plate XXIII, fig. 7), which are 

 caused by the coalescing of two ribs on the inner area. The suture-line, so far as 

 can be seen, is also distinct, especially in the disposition of the inner lobes. 3 

 Therefore I do not think that it belongs to the genus Sonninia, and I hold to my 

 original determination. So close is the resemblance, however, that I took the 

 suture-line (Plate XXII, fig. 34) from the Sonninia figured in Plate XXIII, 

 figs. 7 and 8, to place it as the suture-line of Lillia sulcata ; but on further investi- 



1 See p. 129. I derive Sonninia from some ancestor common also to Amaltheus and Pleuro- 

 ceras, while Lillia comes from Arietites. If we consider that the paths of these two genetic series have 

 been widely different since the time of the Lower Lias, we cannot fail to be surprised at this 

 convergence. It will thus be seen why it is so necessary to insist upon such apparently small 

 differences, and why rudiments are of such importance. How these results have been arrived at, 

 and how the degenerate Sonninia, figured Plate XXIII, figs. 7, 8, is linked to Sonninia Sowerbyi and to 

 Pleuroceras, must be left for the future parts of this Monograph to demonstrate. 



2 See explanation of Plate XXIII, fig. 7, concerning rudimentary spines in Sonninia. 



3 The sutures of larger Ammonites are infallible guides ; but among smaller specimens their 

 interpretation becomes difficult. Not only are the peculiar characters in such cases less developed, 

 but the smaller the specimen the nearer the approach to the simple primordial suture-line inherited 

 by all Ammonites in common. Just in the same way we find the fry of the most different adults 

 to be alike. 



