XXXIV 



INFERIOR OOLITE AMMONITES. 



associated with it. But it is desirable that that should not affect the selection 

 when once made. The difficulty is to identify opalinus. 



There are a number of platyleptogyral, striate Ammonites which have been 

 regarded as opalinus: the figures by different authors bear testimony to that. 

 They are all very like one another; but, first, there are differences in umbilication. 

 All the latumbilicate forms may be at once excluded, for opalinus is angust- 

 umbilicate, — in fact, practically subconcavi-umbilicate. Secondly, examination of 

 the remaining series reveals differences in the curve of the radial line. 



Of the radial line there are at least two distinct types, as shown by the 

 figures appended. Suppl., Fig. 1, is from a Cotteswold specimen figured as 

 opalinus. It has a longer peripheral projection and a rather straighter course on 

 the lateral area than Fig. 2, taken from a Dorset specimen which has hitherto been 

 supposed to be the same species. 



ViQ. 1. — Radial line of Cypholioceras opaliniforme. 



Fig. 2. — Radial line of Lioceras opalinum. 



Fig. 3. — Outline of mouth-border of Cypholioceras opaliniforme. 



Fig. 4.— Outline of mouth-border of Lioceras aff. part it u m. 



Which of these two types agrees with Reinecke's opalinus it is impossible to 

 say, for the radial curves in his drawing are obviously incorrect. So far as that 

 matter is concerned Reinecke's figure must be put out of court. It is right, then, 

 to take as type of opalinus the next specimen so identified, provided that it other- 

 wise agrees with Reinecke's example. This happens to be Quenstedt's, in 

 " ( !eph.,' pi. vii, fig. io. It has a radial line agreeing with that of Fig. 2. 



This, then, is the position arrived at. Opalinus is the type species of Lioceras, 



