GRAMMOCERAS. 161 



p. 36, Plate A, fig. 10) in the fewer number of auxiliary lobes, the less developed 

 accessory lobe in the siphonal and superior lateral saddles, and in less ornamenta- 

 tion altogether. In general outward shape and ornamentation the two species are 

 very similar; but in Gramm. subserrodens the ribs are not quite so sigmoidal as in 

 Lioc. opalinum, the umbilicus is generally more open, and the ventral area is sharper. 



To some members of the genus Haugia certain Grammocerata bear resemblance 

 — Grammoceras dispansum (Lycett) and Grammoceras fallaciosum having at times 

 been considered as varieties of Haugia variabilis ; but Grammoceras may be 

 distinguished from Haugia by the much longer ventral projection of the ribs, the 

 more acute angle which those ribs form with the carina, and the simple character 

 of the suture-line. Gramm. dispansum has no rounded knobs on the inner area like 

 Haugia variabilis, but short club-like protuberances due to the fasciation of the 

 ribs. 



We need not be surprised at Grammoceras presenting some features causing it 

 to converge towards certain members of any of the above-mentioned genera ; 

 because they all belong to the same family Hildoceratidge, and having sprung from a 

 common, and practically speaking, not very remote ancestor (p. 133) are the 

 inheritors of similar characteristics destined to manifest themselves when condi- 

 tions may be favourable. It is otherwise, however, with the genus Dumortieria, 

 which has, in point of descent, had nothing to do with the Hildoceratidse nor with 

 their forerunner Arietites — in fact it has been perfectly distinct, at any rate from 

 the lower portion of the Lower Lias onward. 



Dumortieria and its ancestors have been placed by Dr. Haug in a family, 

 Polymorphidx ; and it is very singular to find that, in the Jurense-zone, some of 

 the species of Dumortieria begin to exhibit such a marked convergence towards 

 Grammoceras. 



In the Jamesoni-zone we meet with Grammoceras antiquum with its com- 

 pressed sides and large carina. About the same horizon, or a little lower, we 

 meet with the small species Polymorphites polymorphus (Quenstedt, not Wright) 

 a shell with small, evolute, uncarinate whorls, and a very simple suture-line. No 

 two Ammonites could be more dissimilar. From Polymorphites comes Dumortieria 

 Jamesoni (Sow.) ; while another branch gives rise to Dumortieria Vernosae (Zittel) 

 in the zone of Amaltheus margaritatus. A slight compression of the sides of this 

 species, a slight decrease in evoluteness, and the advent of a very small carina on 

 the ventral area, gives us Dumortieria Levesquei in the Jurense-zone} 



1 For working out the descent of the genus Dumortieria we are indebted to my friend Dr. Haug 

 (" Polymorphidge," ' Neues Jahrbuch f. Mineral, ' Bd. ii, 1887). It appears to me that Dr. Haug 

 derives Bum. Levesquei directly from I)u7n. Jamesoni ; but I do not see how or why in that case the 

 complex suture-line of Bum. Jamesoni should have become reduced to the simpler suture-line oi Bum. 

 Levesquei. I do not know the suture-line of Bum. Vernosds, which might throw some light on the 



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