part 4] JUEASSic cheonologt : lias. 291 



[Since this paper was read various other items of Gleviceras 

 belonging or akin to the present species have come to hght, and 

 are of particular interest. 



A large septate specimen from the same place as the holotype, 

 and like it only a half, has, however, stouter proportions : it is^ 

 perhaps, more phylogerontic. It shows about a quarter- whorl 

 more ; but, what is chiefly interesting is, that the mark of the 

 inner margin indicates that another complete whorl at least must 

 have been present.^ As the specimen now measures 204 mm. it 

 must, judging by the proportion of radius to diameter as well as bj^ 

 curvature, have been a giant of some 450 mm. diameter. But this 

 inner margin line shows excentrumbilication ; for, whereas about 

 80 per cent, of the whorl was enveloped at one time, in a whorl this 

 has decreased to about i3i^ per cent., and in less than another half- 

 whorl to about 50 per cent. From this specimen the following 

 proportions may be ascertained : — 



ri40, 52, 35, (16?). 



S.^ 204, 51, 33, 19. 



1.450, 45, — , 25. 



The umbilicus in old age returns to the somewhat open condition 

 of youth, which is possessed by the young form described in the 

 following paragraph. 



The next specimen, quite a young example, is also from the same 

 place ; envelopment is about half the whorl ; ribs are regular, with 

 occasional bifurcation ; there are no signs of auriculoids on the 

 exposed part of the whorl, but there is evidence that they existed 

 in the young stage : the line of the inner margin does not follow 

 a regular curve, but is angular (hexagonal in a turn), displaced at 

 intervals, thrown out of its course, by the presence of the auriculoid 

 excrescences — something similar may be noted in the umbilical 

 margin of spinous species. 



This example, which is depicted in PI. XXVII, fig. 3 ; PI. XXIX, 

 fig. f3 ; and PI. XXX, fig. 4, gives the following proportions : — 



Q r29, 49, 29, 24, -K.2 



^•131, 48, 27, 22, +K.2 » 



The next specimen is from Charmouth, a 'pyritised Oxyno- 



^ In the course of these investigations I have made a discovery which may 

 be of importance for the identification of types. In my father's descriptions 

 of ammonites, 'Geology of Cheltenham' new ed. 1844 (1845), the diameter 

 given is not that of the specimen as preserved, but is the assumed diameter of 

 the perfect shell. Thus, in his description of Aininonites gracilis (p. 104), the 

 diameter is given as ' about 9 in.' The specimen actually measures 145 mm. 

 (5|, inches) in diameter, but is fully septate and carries indications of another 

 half- whorl at least, shown by remains of inner edge. The diameter of 9 inches 

 is obviously reckoned to include this extra half-whorl or so, but is, perhaps, 

 rather an overestimate. This method of giving diameters, now that it is 

 discovered, clears up many discrepancies in these descriptions. It may be 

 that other authors have adopted this plan, which might easily raise doubts as 

 to the identity of types. There is no possible question about the identity of 

 A. gracilis. 



^ Without keel, with keel. 



2a2 



