32 INFERIOR OOLITE AMMONITES. 



agree with the umbilicus as exhibited by smaller specimens of that species and is 

 therefore likely to mislead. A comparison of Lioc. simile with Lioc. giganteum of 

 the same diameter, shows a much deeper, narrower, and smoother umbilicus, a 

 more compressed ventral area, a greater thickness of section at the inner area, and 

 the test far smoother in texture, with smaller ribs which disappear early, while it 

 is seen that specimens of Lioc. giganteum carry the opposite characters throughout 

 their growth, the ribs, especially, being visible in the later as well as in the earlier 

 portions of the umbilicus. The umbilicus of Lioc. simile is certainly different to that 

 of the large or small specimens of Lioc. giganteum, appearing to possess fewer coils, 

 and not to have that somewhat excavated appearance which the umbilicus of 

 specimens of that species has. 



Between Lioc. decipiens and its variety Lioc. simile in the S owerby i-zone there 

 exists a relationship similar to that between Lioc. bradfordense and its variety 

 Lioc. giganteum in the Murchisonaj-zone, and to that, which, however, seems stronger, 

 between the thick and thin forms of Lioc. opalinum in the zone of that name. 

 Comparing the specimens from these three zones it seems not unlikely that the 

 first were derived from the two forms of Lioc. opalinum, to which species they 

 have much resemblance in smooth character of test, small umbilicus, &c, but 

 from which they differ in the shape of ventral area and carina; while, on the other 

 hand, the second have rather more the appearance of exaggerated forms of 

 Lioc. elegans, Young (not Sowerby). In this case we have to suppose, first, 

 the existence of links in the Murckisonce-zone, and, secondly, of links in the 

 Jurense- and Opalinum-zones which we do not at present possess. Another theory 

 might be that Lioc. decipiens and Lioc. simile had been derived from Lioc. opalinum 

 through Lioc. bradfordense and Lioc. giganteum, but then we must suppose that 

 Lioc. opalinum gradually changing became ribbed and took on a different method 

 of coiling and a different form (bradfordense), and that next those forms reverted 

 to a method of coiling and ribbing similar to that at first possessed by L. 

 opalinum (decipiens). I scarcely favour this view; but, if the former be correct, 

 we ought to describe our specimens as Lioc. opalinum mut. decipiens and Lioc. 

 opalinum mut. simile, or Lioc. opalinum mut. sp. ?, mut. decipiens or simile, — an 

 innovation which, though very probably expressing the true state of the case, our 

 present knowledge hardly warrants our adopting. It is, however, interesting to 

 bear these conclusions in mind, for it perhaps tends to demonstrate that any 

 attempt to class Lioc. decipiens and Lioc. simile as smooth varieties of Lioc. brad- 

 fordense and Lioc. giganteum, would be quite unadvisable. 



The probable position of the specimen is the Sowerbyi-zone near Sherborne, but 

 I am not able to speak with certainty. It must be a very scarce form. Plate XV, figs. 

 1, 2, give a very good idea of a well-preserved specimen with just the commencement 

 of the mouth-border, and with the last of the fine ribs before it became quite smooth. 



