428 INFERIOR OOLITE AMMONITES. 



From Sonn. inasqua it differs by being less costate, less carinate, and less 

 umbilicate, though the umbilicus is more excentric. 



In septation this species seems to conform to what would be expected of the 

 development of 8. subirregularis ; in external appearance it gives very little guide 

 to its genealogy. 



8 own. sub simplex occurs in the Goncavum-zone of Bradford Abbas, and cannot 

 be called a rare species. It is figured in PI. XCV : fig. 6, side view ; fig. 7, outline 

 front view, both reduced one-half ; fig. 8, septa. 



D. 1 The lobes are decidedly long, and there is little degeneration of lobes or 



lobules in phyletic development. 



A. The lobes are fairly developed. 



Fig. 52. — Outline of L of Sonn. eostigera. Fig. 53. — Outline of L of Sonn. obtusiformis. 



Sonninia costigera, 8. Buclcman. Plate CII, figs. 1 — 3. 



Discoidal, compressed, carinate. "Whorls ornamented after the regular 

 spinous stage, 2 with costae and nodicostaa — as 3 to 1 — to about 60 mm. 

 diameter, later with numerous, direct, reclining, closely-set, rather small, often 

 connate costae ; later with larger, more distant, undulate, very feebly bullate, 

 direct, somewhat reclinate, separate costae. Ventral area rounded, somewhat 

 flattened medianly, divided by a small, rounded hollow carina. Inner margin 

 steep, ill-defined. Inclusion not two-fifths. Umbilicus concentric, graduate, with 

 slightly gibbous whorls. Whorl-section somewhat obovate. Suture-line with a 

 somewhat long, symmetrical superior lateral lobe, the terminal lobule axial, but 

 slightly anisosceloid and inequicellate ; the lateral lobules nearly isometric, not 

 quite opposite. 



1 The stocks included in this division seem also to be characterised by a tendency to enlarge their 

 costae after a period of decline, even so much as to become spinous. 



2 This stage lasted apparently to about 35 mm. diameter, but the centre is badly preserved, and 

 has been damaged by the tool. 



