430 INFERIOR OOLITE AMMONITES. 



terminal lobule of L might have favoured the placing of costigera in one of the 

 earlier groups. 



Sorm. costigera is a scarce fossil in the Concavum-zone of Bradford Abbas. 

 It is represented in PI. Oil : fig. 1, side view, reduced to three-fourths of natural 

 size ; fig. 2, front view, similarly reduced ; and part of the suture-line, fig. 3. 



Sonninia obtusipormts, S. Buchman. Plate XC, fig. 10 (Plate LXXXV, figs. 7, 8 ?). 

 See Plate LXXII, figs. 3—5, and page 333. 



What is presumably a young form of this species has been placed in PI. 

 LXXXV, figs. 7, 8. The suture-line of the type has been re-drawn in PI. XC, 

 fig. 10, for comparison with that of Sown, palmata (see p. 374). It may be 

 reasonable to derive the pentadactyloid L of this species from the tridactyloid, 

 incipiently tetradactyloid L of costigera — there is similar furcation in the inner 

 lobule of L of both species. To this end it must be supposed that the terminal 

 lobule of the cos%era-development became more intra-axial and more anisosceloid. 



B. The lobes are remarkably developed, and the septa are much interlocked. 



Fig. 54. — Outline of L of Sonn. renovata. 



The above heading applies to a group of species which are remarkable for 

 their very long, closely interlocked lobes. By their septation they are distin- 

 guished from other species of similar external form, which mostly possess rather 

 short lobes. This is strikingly illustrated by the figures in PI. XCIV, which 

 show two species very similar in shape, possessed of remarkably different septa. 



The following fossils fall into at least two series ; to understand them it is 

 necessary to draw attention to the biologically-earliest species, Sonn. locuples. 

 Under this name are grouped for the present two forms which show only 

 slight external differences from one another : Sonn. locuples /3 is coiled fractionally 

 closer to the spines than is the type, Sonn. locuples a. In septation there is one 



