150 INFERIOR OOLITE AMMONITES. 



enfilee et moins haute, par ses lobes et par I'absence d' ornaments sur ses premiers 

 tours." 



The regularity and direction of the ribs are the chief characters. The nearly 

 equidistant ribs are close together and are not bunched as in H. variahilis ; but 

 the especial distinction is that they proceed from the tubercles in the same direc- 

 tion as a line would, if drawn from the centre ; while the ribs of H. variahilis 

 would fall behind such a line. As regards the carina, I cannot say very much, 

 since I have not had sufficient material of H. variahilis to compare; and the 

 absence of ornaments on the first whorls seem also to be found in R. variahilis, 

 though it is, perhaps, not so noticeable. However, there is one great difference 

 between the two species which was not mentioned by Dumortier, and this is 

 in the coiling of the umbilicus. All my specimens of this species show a quickly- 

 coiled umbilicus; that is, they reach a given diameter with fewer turns than does 

 Haugia variahilis. A comparison of d'Orbigny's reduced figure (all measurements 

 doubled) and the large specimen of Haugia jugosa (PL XXIV) shows that the 

 latter has reached the same diameter as the former in about half a turn less. The 

 following table will show the difference between the two. 



Diameter 



Width of total umbilicus 



Width of umbilicus, one whorl back 



Width of umbilicus, two whorls back 



A comparison of figs. 11, 14 in Plate XXIII, with fig. 2 in Plate XXV, will 

 show the difference in the umbilicus of the young forms ; singularly enough, the 

 umbilici of the adults become more nearly equal in size if the same diameter be 

 considered, but not if the same number of turns be taken. 



It seems probable that the species is a mutation of Haugia variahilis — a 

 descendant assuming the senile stage at an earlier period ; but I cannot say if this 

 be correct without an extended study of the suture-lines, which my material does 

 not allow. Dumortier says (p. 79) that the lobes are coarser and not so much 

 branched as in H. variahilis. If this be so in similar-sized specimens it would tell 

 against this idea of their relationship, and we should have to seek for the ancestor 

 of this species in H. navis or H. malagma. On the other hand, however, the 

 specimen figured by Wright, pi. Ixvii, figs. 1, 2, appears, from its ribs being directed 

 slightly backwards, as if it might be an intermediate form between H. variahilis 

 and the true H. jugosa, in fact, the parent of the latter and the descendant of the 



' For the sake of comparison the same amount has been measured off on the specimen of 

 S. jugosa. 



. variabilis. 



H. jugosa. 



Inches. 



Inches. 



7-4 



7-4^ 



2-7 



2-6 



1-4 



1-05 



0-7 



* 1 1 r» rfc 



0-45 



