SUMMARY PLATE XIII. 



Agas. lsevigatum. Fig. 1 shows the more compressed variety of this species. 



Ast. obtusum. Fig. 2 shows the stouter variety with well marked channels (a little too 

 deeply shaded), with stout gibbous whorls and broad abdomen. This has young almost identical 

 with the adults of the stout varieties of Agas. hevigatum. 



Ast. Turneri. Fig. 3 shows typical variety, with flattened sides and deep channels. It is 

 notably more involute than obtusum. 



Ast. Brooki. Fig. 4 shows an extreme involute variety of this species, with very conver- 

 gent sides and narrow abdomen. The channels are almost obliterated, and the keel very 

 prominent. 



Ast. Collenoti. Fig. 5 gives a view of this remarkable dwarfed form, in which degenera- 

 tion of the pilte and the channels and convergence of the sides have produced morphological 

 equivalence with Oxyn. oxynotum and Guibali. The amount of the involution is, however, 

 greater than in any preceding species of the same series. 



Agas. striaries. Fig. 6 gives a view of the adult, with a decided keel. 



Agas. Scipioniarmm. Fig. 7 shows the stouter, heavily tuberculated variety, which has 

 young almost identical with the stouter varieties of Agas. striaries. 



Agas. Scipionis. Fig. 8 shows an aged specimen in the Museum of Comparative 

 Zoology, with extremely involute whorls, but keel still prominent. The degeneration of the 

 adult as regards the pilae and form can, however, be inferred from this figure. The old of 

 Sdpionianum at the same age is much less changed, and does not exhibit increased involution 

 of the whorls. 



Oxyn. oxynotum, Fig. 9, 10. The first figure shows the young of a variety in which at 

 an early stage there is close likeness to the young of Agas. striaries, and the adults of Agas. 

 Icevigatum. 



Oxyn. Simpsoni. Fig. 11 shows the stouter form and slightly greater involution of the 

 whorls in this species when compared with oxynotum. 



Oxyn. Lymense. Fig. 12 shows the greater involution of whorls as compared with any 

 preceding form of the same subseries, and the very acute degenerate whorl. 



Oxyn. Greenoughi. Fig. 13 shows the stout form of the whorls better defined, and pilse 

 of this subseries as compared with the oxynotum subseries. 



Oxyn. Guibali. Fig. 14 shows more involute whorls than in Greenoughi. 



Oxyn. Lotharingum. Fig. 15 shows the smaller size of this species, and the degener- 

 ation of the pilae. The involution of the whorls is, however, greater than in any preceding 

 species. 



Oxyn. Oppeli. Fig. 16 shows the extremely involute form of the Middle Lias. The stout 

 whorls indicate that no great amount of degeneration had taken place. It may have been a 

 direct descendant of Greenoughi. 



