﻿3.-2-1 



INFERIOR OOLITE AMMONITES. 



thickened, and making a considerable constriction on the core ; but, if it be the 

 mouth, the body-chamber is very short, only three-eighths of a whorl. 



The second of the large fossils is the type of dominans, figured, 302 mm. in 

 diameter. The third is a form which links the type, or more correctly dominans /3, 

 with modesta /3 (see next page). It is 312 mm. in diameter, incomplete, with 

 only a quarter-of-a-whorl body-chamber. It has a very excentric umbilicus, 

 129 mm. in diameter, but at one whorl back only 47 ; yet the excentricity does not 

 begin so early as in modesta. It is rather smooth till nearly adult, and then 

 shows somewhat coarse distant ribs, which, however, are not large. 



This species has been obtained from Bradford Abbas, Halfway House, &c, in 

 North Dorset, and from Chideock in South Dorset. In PI. LXVI are shown the 

 side view and outline front view of a splendid specimen, reduced about two-thirds. 

 PI. LXVII, figs. 1, 2, illustrate the two views of a young form to compare with 

 young margmata. In PI. LXIX the very distinct close-ribbed variety is delineated. 

 A form intermediate in certain respects between marginata and dominans is 

 illustrated in PI. LXVII, figs. 3, 4, 5. 



Sonninia kevirescens, S. Buckman. Plate LXX, fig. 1. 



Discoidal, compressed, carinate. Whorls, in section, sub-oblong, ventrally- 

 convergent, ornamented with inconspicuous, closely-set, undulate, direct, upright, 

 ventrally-inclined ribs, 1 followed by distant, coarse ribs, which increase considerably 

 in size with age. Ventral area subacutely arched, bearing a small carina. 2 Inner 

 margin fairly upright, well-marked, subconvex. Inclusion about one-fourth. 

 Umbilicus becomes excentric in correlation with the larger ribs. 



This is not merely a smaller edition of dominans a — it is more than that. The 

 ornaments of the inner whorls are somewhat less pronounced, while the coarse 

 ribs begin much earlier, are more distant, and attain a larger size. Not even in 

 the costate forms of dominans do the adult ribs become so large ; and those forms 

 certainly do not show such a difference in the ribbing of the inner and outer 

 whorls. 



Reduction of ornament is a feature of the genetic series from crassispinata 

 onwards. This reduction is manifest in the individual earlier and earlier as the 

 species are followed up, and it is succeeded by an elaboration which begins with 



to have been parallel to the ribs and growth-lines, and the ventral lappet is obtusely rounded. The 

 body-chamber in this case is half a whorl in length. 



1 A few rudimentary spines are irregularly placed on the ribs of the central whorls. 



2 The carina is apparently not hollow in the adult ; possibly it is in the young. 



