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INFERIOR OOLITE AMMONITES. 



Sonninia subcostata, S. Buclcman. Plate LXXI, figs. 4, 5. 



Discoidal, compressed, carinate. Whorls, in section, compressed oblong, 

 ornamented with small, closely-set, direct, upright, ventrally-inclined ribs. (The 

 spinous stage is very rudimentary, and evidently ended at about 20 mm. diameter.) 

 Ventral area narrow, arched, divided by a small, rounded, presumably-hollow 

 carina. Inner margin conspicuous, smooth, upright, flat. Inclusion about two- 

 fifths. Umbilicus regularly graduated. 



This shell seems to me exactly the development which might be expected from 

 submarginata, from which it differs by being much more finely ribbed, and by its 

 rudimentary spinous stage. It is the morphological equivalent of Sonninia 

 modesta a, but is thinner and possesses a larger umbilicus, which the following 

 measurements will indicate : 



modesta. subcostata. 



Diameter . . .129 mm. ... 129 mm. 



Umbilicus . . 34 „ ... 46 „ 



Umbilicus, one whorl back . 16 ,, ... 20 ,, 



Umbilicus, two whorls back . 7 ,, ... 10 ,, 



This form occurs at Bradford Abbas, but is rare. A side view is shown in 

 fig. 4, Plate LXXI, and the aperture in outline, fig. 5. 



Sonninia substriata, 8. Buckman. Plate LXX, figs. 6, 7 ; Plate LXXI, figs. 6 — 8; 



and an intermediate form, Plate LXXII, 

 figs. 1, 2. 



Discoidal, compressed, partly carinate. Whorls, in section, oblong, only those 

 of the centre ornamented with small, direct, ventrally-inclined costa3, the spinous 

 stage being absent. Outer whorls marked only with sub-direct, laterally and 

 ventrally-inclined growth-lines. Ventral area rounded, divided by a small keel, 

 which becomes more and more rudimentary with age, and is entirely lost at about 

 135 mm. (5^ inches) diameter. Inner margin well-marked, flat, becoming more 

 sloping with age. Inclusion, in youth one-half, decreasing to about § in older 

 shells. Umbilicus with a slight excentric tendency. 



This form is the descendant of subcostata, from which it is easily distinguished 

 by the loss of ribs, loss of carina, and smaller umbilicus. It is the morphological 

 equivalent of simplex, but is distinguished by its slower-coiled centre, more con- 

 centric umbilicus, by possessing ribs to a later date, and by being thinner. The 



