396 INFERIOR OOLITE AMMONITES. 



The above description applies only to the type-form depicted in Plate XCVI, 

 figs. 3 — 5. It is necessary, however, to remove from Sonn. marginata the large 

 specimen figured in PI. LXIV on account of differences in septation ; and it is 

 assumed that that form is the adult of regularis. Such differences as there are in 

 septation may be supposed to be due to difference in age. This, however, cannot 

 be assumed in regard to that large specimen and Sonn. marginata, because con- 

 tinued compression of the whorl favours the change from a symmetrical to an 

 asymmetrical L ; whereas a contrary change, from marked asymmetry to greater 

 symmetry, would have to be conceded, if the large specimen in PL LXIV be 

 considered to be the same species as Sonn. marginata (PI. LXII). 



The type Sonn. regularis differs from Sonn. marginata externally by its more 

 prominent, altogether more massive carina, by its thicker whorls and its stronger 

 spinous stages. Internally, as to septation, the differences are still more 

 marked, for the septa of Sonn. marginata, which have been exposed since the type 

 was figured and are now placed in PL XCVI, fig. 6, for comparison, show 

 asymmetrical L with distinctly intra-axial, markedly anisosceloid, and inequi- 

 cellate terminal lobules. 



Sonn. regularis occurs in the Concavum-zone of Bradford Abbas. The side 

 view of a fairly preserved shell with test present is depicted of natural size in 

 PL XCVI, fig. 3 ; its whorl-section, with part of embraced whorl, in outline, fig. 4 ; 

 the portions of two septa, fig. 5. 



Sonninia parviuostata, S. Buckman. Plate CIII, fig. 22. 



See Plate LXXV, figs. 3—5 ; Plate LXXIV, figs. 2, 3, and page 339. 



It is a long step from Sonn. regularis to this species ; the only bond between 

 them is the similarity of the septation to that of Sonn. regularis, adult {antea S. 

 marginata), PL LXIV. The likeness in certain respects between Sonn. costata 

 and this species would warrant their connection ; but certain differences in septa- 

 tion necessitate their separation in this scheme. These species, however, seem to 

 be near relations ; but the evidence of the septa appears to suggest that the genetic 

 connection is not so direct as was assumed when they were first described. 



