418 INFERIOR OOLITE AMMONITES. 



The type agrees with Bonn, irregularis in septation, but the pronounced 

 pairing and connation of its strongly reclinate costse prevent the species being 

 regarded as directly connected genetically. Sonn. irregularis gives signs of having 

 produced a form with rather upright single ribs. 



B. The lobes become short in catagenesis. 



Fig. 47. — Outline of L of Sonn. ■ptycta. 



a. Costas weak. 



Sonninia. spinifera, 8. Buckman. Plate C, figs. 1 — 4. 



See Plate L, figs. 14, 15; Plate LXXIV, figs. 4— 6, 1 and page 335. 



The description of the young and immature shells has been already given. 

 The adult shell has since been identified, and it has another whorl, which shows 

 rather broad, inconspicuous, direct ribs, practically obsolete on the outer third. 

 The ribs are very feebly bullate in their middles, rather variable in size and 

 slope, tending to become more upright, and much more distant with age ; in fact, 

 there are only about half as many ribs on the last half-whorl as on the last half of 

 the preceding whorl, and, in proportion to the size of the shell, they are less 

 conspicuous. The ventral area is rounded ; the carina is small. The inner margin 

 becomes more defined with age and slightly subconvex. The suture-line is 

 somewhat simple ; the superior lateral lobe is asymmetrical, owing to the 

 anisometry of the lateral lobules ; the terminal lobule is intra-axial, very 

 anisosceloid, very inequicellate. 



This adult specimen, which is more than a whorl larger than the one previously 

 figured, makes an important contribution to our knowledge of the species. It is 

 seen that there is a tendency towards less costation with age, the ribs produced 

 are fewer in number, for in places they are wanting, somewhat irregular, and in 



1 An uncorrected error gives " spinigera " in the explanation of Plate LXXIV. This should be 

 altered. 



