414 INFERIOR OOLITE AMMONITES. 



Sonntnia brevispinata, 8. Buchman. See Plate LXXV, figs. 6 — 8, and page 343. 



This species was supposed to be connected with magnispinata by means of 

 semispinata ; but the latter has different septation, and has now been removed. 



2. Ribs fairly upright. 



Sonninia mutans, 8. Buchmcm. Plate XCI, figs. 4 — 6. 



Discoidal, compressed, hollow-carinate. Whorls ornamented with regular 

 spines to about 13 mm. shell-radius, with spinicostas and costas 2 to 1, later 

 1 to 4, to about 47 mm. radius ; with bullicostse to about 70 mm. radius ; lastly, 

 with direct, not prominent, not distant, nearly upright, ventrally-inclined costa?. 

 Ventral area rounded, the median part slightly depressed, divided by a small, well- 

 defined, rounded, slightly impressed hollow-carina. Inner margin slightly defined 

 until the bullicostate stage ; later more defined, slightly convex, nearly upright. 

 Inclusion about two-fifths. Umbilicus open, concentric, with gibbous, spinous 

 whorls. Whorl-section quadrate-oblong. Suture-line with asymmetrical L, its 

 terminal lobule anisosceloid, intra-axial, its lateral lobules nearly isometric, non- 

 partite, not opposite. 



By its external form and ornamentation this species would seem to be the link 

 between spinosa and quadrifida (p. 394) ; but its suture-line, showing an asym- 

 metrical lobe with intra-axial and isosceloid terminal lobule, indicates considerable 

 difference. These characters of the suture-line are like those of the paucinodata- 

 group (pp. 370 and 408), but the L is narrower stemmed; further, the ornament does 

 not allow it to be placed in this connection — the spines are too numerous and too 

 strong for a descendant of nodata. It is in these characters that it differs specifi- 

 cally from that species, and it shows, further, more upright ribs, more quadrate 

 whorls, and a well-defined inner margin. 



The characters of ornament and suture-line show the greatest affinity, in a 

 genetic sense, with Sonn. magnispinata (pp. 341 and 413), but it cannot be placed as 

 a descendant of that species because its ribs are far less reclinate throughout life. 

 Its present position, then, as a collateral of magnispinata, but a step later in 

 development, seems the only one possible. 



8onninia mutans occurs in the Concavum-zone at Bradford Abbas. 



