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INFERIOR OOLITE AMMONITES. 



morphological representation. The immature form (PI. LIX, figs. 2, 3) shows 

 that this is very probable. It differs from crassispinata a (PI. XLVIII, figs. 16, 

 17) in having much coarser spines in the inner whorls ; that is to say, the coarser- 

 spine-period of crassispinata has been forced further back in this form by earlier 

 inheritance. Sonn. acanthodes a (adult) shows the further operation of the same 

 law, for practically it omits the irregularly spinous stage which should separate 

 the two regular spinous stages. 1 



These forms of Sonn. acanthodes are not derived from crassispinata ft, because 

 in the adult stage of that form the bifurcate spinous ribs are associated with a 

 whorl flatter-sided than that which accompanies the same ornamentation in either 

 of the " acanthodes " forms. From acanthodes a a short separate branch can be 

 traced ; but I must leave it until I have described the more important crassi- 

 spinata-sto ck. 



Sonn. acanthodes occurs in the Concavum-zone of Bradford Abbas, Dorset. The 

 type-form of the species is figured in PI. LX, side view, and in PI. LXIII, fig. 1, 

 front view. In PI. LVIII the side view of the form ft is shown, and the back view 

 with its coarse prominent ribs is depicted in PI. LIX, fig. 1. Two views of a 

 young form of the species are given in PI. LIX, figs. 2, 3. What the younger 

 stages of this species were like can be gathered from the figures of the early 

 stages of crassispinata (PI. L, figs. 19 — 22). 



Sonninia irregularis, S. Buchnan. Plate LXI. 



Discoidal, compressed, hollow-carinate. Whorls compressed, subquad- 

 rangular, rather flattened laterally, ornamented, when adult, with nearly upright, 

 ventrally-inclined ribs, sometimes carrying median spines, beyond which the ribs 

 often bifurcate. Ventral area broad, hollow carina well-marked. Inclusion one- 

 fourth. 2 Inner margin well-marked, deep, and very slightly convex. 



There are three varieties of this species : 



a (PI. LXI). With nearly upright ribs. 



ft. Spines end earlier; reclining ribs. 



■y. A more involute form, with an overhanging inner margin. Its ornamen- 

 tation is much less pronounced, though its irregular spines are persistent to a 

 fairly late age. Its ribs are slightly reclining. 



1 Compare the various specimens of crassispinata for the shortening of the intermediate irregular 

 stage and the tendency to join the regular stages. 



2 This does not agree with the side view ; but the plate is drawn from a photograph, and it 

 illustrates the difference between monocular and binocular vision. The reason for the incorrectness 

 of the photograph will be apparent to anyone conversant with the action of a lens. 



