SUMMARY PLATE XII. 



Psil. planorbe, var. leve. Fig. 1 shows the dwarfed, but very common form of this 

 variety, which is evidently a close ally of the next, Am. miserabile. 



Am. miserabile. Fig. 2 represents the acute but keelless variety of this species. 



Am. obtusiforme, Fig. 3, is not very similar to viiserabile, but the evidence of gradations 

 places it in this relation. 



Am. semicostatum. Fig. 4. The figure represents the nearly full-grown shell, but it may 

 be easily seen that, if the keel were absent, the smooth whorls of the young would closely resem- 

 ble the adult whorls of Psil. planorbe, var. leve. 



Am. Hartmanni, Fig. 5, exhibits young and adult characters like those of the preceding. 



Am. tardecrescens. Fig. 6, belongs to another subseries of forms than that in which it 

 is placed, but it serves to show that quadragonal whorled shells with channelled abdomens 

 existed in this genus. 



Am. Bodleyi, Fig. 7, shows a slightly degenerate compressed whorl, and is the terminal 

 form of the subseries containing Hartmanni. 



Am. kridioides. Fig. 8 gives a view of the transition between Arnioceras and the lowest 

 species of Coroniceras. The smooth young straight pilee and divergent sides of the adult whorl 

 are clearly shown. 



Cor. kridion. Fig. 9 shows that the tuberculated pilse, smooth young, and form of whorl 

 are exactly intermediate between Am. kridioides and Cor. Sauzeanum. 



Cor. Sauzeanum. Fig. 10 shows the later nealogic and ephebolic stages, having the 

 peculiar divergent sides, flattened abdomen, and prominent tubercles of a typical coroniceran 

 form. The young, however, still retain the smooth aspect, indicating derivation from Arnioceras. 



Cor. bisulcatum. Fig. 11 shows the very deep channels and peculiar flat abdomen of this 

 species. 



Cor. rotiforme. Fig. 12 represents a form similar to Cor. coronaries. 1 



Cor. Lyra, Fig. 13. This is as a rule much smaller than rotiforme. The sides are more 

 convergent, and the whorls more compressed and less numerous than in that species. 



Cor. G-muendense, Fig. 14, shows degeneration in the compressed whorls of the adult as 

 compared with Lyra. 



Cor. trigonatum. Fig. 15, exhibits still more clearly than in Gmuendense the effects of the 

 premature development of old age characters. The early stage at which they appear is repre- 

 sented in the widening of the spaces between the pilae. This shell was inferior in size only to 

 rotiforme, and much larger than Gmuendense. 



Cor. latum, Fig. 16. This is the extreme form of this genus, and has in proportion to 

 its age -and size more divergent sides and a broader abdomen than any other. It may be, as 

 described in the text, the young of a yet undiscovered adult, but is not the young of the next 

 form, Bucklandi. 



Cor. Bucklandi. Fig. 17, shows the very stout bulky form of this species, its pilse, and 

 the divided pilre of the early stages of the sinemuriense variety. This species is not inferior to 

 rotiforme in size. 



1 The size of the plates did not permit the use of larger specimens, and therefore these figures do not properly 

 represent the comparative size of the different species in the genus Coroniceras. 



