424 INFERIOR OOLITE AMMONITES. 



2. Costas paired. 



Sonninia alternata, 8. Buckman. Plate OIII, fig. 7. 



See Plate LXXVI, figs. 7—9, Plate LXXVII, figs. 3—5, and page 346. 



Add to the description : suture-line with fairly long lobes, L with very ani- 

 sosceloid, inequicellate, intra-axial terminal lobule ; lateral lobules very anisometric, 

 inner lobule much abbreviated, outer lobule feebly bipartite. 



Sonn. alternata is distinguished from Sonn. revirescens by greater asymmetry of 

 L, and by much stronger costse following the spinous stage. 



In spite of external resemblance to 8. biplicata in the matter of paired ribs, 

 an association with biplicata a is rendered doubtful by the septal details. Con- 

 sidering that the course of septal development, consequent on the flattening of the 

 area supported by the outer lobule of L, is to make the support larger, it would 

 be difficult to imagine an abbreviation of the strongly duplicate outer lobule of L 

 in biplicata sufficient not only to reduce it to the proportions of the outer lobule 

 of L in alternata, but to cause the duplication almost to disappear. The tendency 

 of the development is to enlarge the outer lobule at the expense of the inner, 

 whereby the duplication should become stronger, and the reason for the abbrevia- 

 tion of the inner lobule in course of development is seen in the greater approxi- 

 mation of septa which accompanies catagenesis. 



In Sonn. biplicata (i, PI. CIII, fig. 16, the septa are more of the pattern 

 seen in alternata; but it may reasonably be doubted whether the association 

 of biplicata a and |3 in the same species is justifiable. Doubt is thrown on the 

 possibility of biplicata (5 having developed into alternata a, by the difference in the 

 ventral inclination of the ribbing, it is so much greater in the former. 



]3. Inner lateral lobule of L not abbreviated in spinous species (but it is so later 



on in the phyletic series 1 ). 



Fig. 50. — Outline of L of Sonn. subirregularis. Fig. 51. — Outline of L of Sonn. subsimplex. 



1 Abbreviation of the inner lateral lobule of L begins much later in this series than in that 

 arranged under C, a ; and consequently in the smooth form (S. subsimplex) anisometry of the lateral 

 lobules is less marked than in the spinous form of the other series. 



