XX 



INFERIOR OOLITE AMMONITES. 



4. Denckmannia ? * malagma (Dumortier). Suppl., Plate IV, figs. 1 — 3. 

 187-1. Ammonites malagma, Dumortier, PL xxii, fig. 1 only. 



Description. — Substeno-subleptogyral, latumbilicate, subcrassi-subirregulari- 

 bullicostate, rursi-recti-costate, subalti-septioarinate. 



Remarks. — The co-stse are arranged somewhat in groups of four, whereof two 

 unite into a fairly large bulla, while the other two come close together, are scarcely 

 joined, and have no bulla. Of any four ribs one is distinctly larger than the 

 others, and these larger ribs appear like raised bands across the lateral areas. 

 The largest rib is the first in a series of four, so that the rib-sculpture may be 

 diagrammatically represented thus : 1 2 3 4, the bracket denoting the connate pair. 



Comparison. — With malagma, Dumortier : that author has figured under this 

 name two species, whereof one is much more coarsely costate than the other. 

 It is only with the coarsely costate form (his fig. l) that comparison need be made. 

 This comparison is not easy because the specimen represented is obviously some- 

 what ill preserved, so that irregularity of costation has been exaggerated. But 

 certain points may be noticed, as follow : the diameter of Dumortier's figure 

 without carina is 96 mm., with an umbilicus of 39 mm., one whorl back 18 mm., 

 two whorls back 8 mm. Taking the same diameter en my specimen the umbilicus 

 is respectively 39 mm., 20 mm., 9*5 mm, This indicates that my specimen is more 

 concentrically umbilicate. If Dumortier's specimen be correctly drawn, the differ- 

 ence deserves careful consideration ; but experience tells that exactly correct 

 drawing in such details is very difficult to obtain. 



Further remark may be made with regard to Dumortier's description. He 

 says — " With regard to the bifurcate costas, it may be noticed that the rib which is 

 in front is always the larger (que e'est toujours la cote qui est en avant qui est la 

 plus volumineuse)." This does not agree with my description, but there may 

 be an ambiguity here. Thus of ribs 1, 2 of the bifurcate series, 1 is the larger in 

 my specimen, not 2. as Dumortier's description seems to infer. Now 1 is the rib 

 which appears first from the point of view of growth, but 2 is the rib which is in 

 front, that is, nearer to the aperture. 



Locality and Stratum. — Somerset : Shepton Beauchamp, in the " Upper Lias," 

 just above Hildoceras bifrons. 



Date of Existence. — Variabilis hemera. 



1 The assignment of this and the following species to Denckmannia is provisional. 



