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INFERIOR OOLITE AMMONITES. 



Sonninia ptycta, 1 8. Buchnau. Plate LXXIII, fig. 1. 



Discoidal, compressed, carinate. Whorls, in section, gibbous-sided, subquad- 

 rate, ornamented for the most part with direct, reclining, ventrally-inclined 

 ribs, but the central part of the umbilicus ornamented only with strong spines 

 (to 40 mm. in diameter), later with spines separated by ribs, — the intermediate 

 ribs at first single, afterwards more numerous. Ventral area broad, laterally- 

 gibbous, median portion somewhat depressed, bearing a strong, rounded, presum- 

 ably hollow carina. Inner margin not defined till late, then nearly upright, but 

 upper edge much rounded. Inclusion very little — about one-fifth. Umbilicus 

 with gibbous whorls. Termination, adult, a mouth with merely a plain, thin edge 

 parallel to the ribbing and growth-lines. 



This form is the morphological equivalent of Sonninia marginata, to which it 

 certainly bears considerable resemblance ; but its whorls are more tumid, and the 

 markedly reclining ribs give to them and to the umbilicus a twisted appearance 

 not seen in marginata. It also lacks the pronounced inner margin. 



It bears to acanthodes a the same relationship as marginata bears to irregu- 

 laris, namely, in adolescence it is the morphological representation of the adult 

 acanthodes. It retains the same reclining ribs. It is true that the same feature 

 is seen in irregularis y ; but that form has, like marginata, a peculiarly angular 

 appearance due to the laterally-flattened whorls and the marked inner margin ; 

 it is also spinous a much longer time. 



In order to economise space, I have only given a very reduced figure of this 

 shell. It should be possible for anyone, taking the figure of the morphological 

 equivalent marginata, and bearing in mind the differences noted above, to recognise 

 the species, remembering its genetic position in relation to acanthodes. The reduced 

 figure gives a curious result, — the adult, ptycta, reduced to about one-quarter, is 

 wonderfully like the young cymatera (its descendant) drawn of the natural size. A 

 better illustration of the law of earlier inheritance could hardly have been devised. 



Sonn. ptycta, is a rare form obtained from the Goncavum-zone of Bradford 

 Abbas. It is depicted about one-quarter of natural size in PI. LXXIII, fig. 1, the 

 drawing being a copy from a photograph. 



Sonnintta cymatera, 2 8. Buchnau. Plate LXXIII, figs. 2, 3. 



Discoidal, compressed, carinate. Whorls, in section elliptical, ornamented 

 for the most part with well-marked, direct, somewhat reclining, ventrally-inclined 



1 YItvktos = folded. 



2 Kv/xarripos = waved, 



