160 INFERIOR OOLITE AMMONITES. 



Some of the species with sulcata ventral area were placed by Haug in the 

 genus Hildoceras (group of Hild. Mercati), presumably on account of these sulci.^ 

 It is true that many of the less-developed members of the genus Grammoceras are 

 like such members of the genus Hildoceras as happen to be more developed,^ not 

 only on account of these sulci but also on account of their suture-line and general 

 shape. However, the siphonal saddle of Hildoceras is generally much broader than 

 in Grammoceras, while the inner part of the suture-line — especially the inferior 

 lateral lobes — is less developed. In addition to this, however, there is the dis- 

 tinction in ribbing — the lateral bend in Grammoceras being so much less con- 

 spicuous than in Hildoceras, a difference well shown in figs. 30 and 45 of Plate A. 

 In spite of their sulcate ventral area I do not think these species of Grammoceras 

 have anything to do with Hildoceras genetically. 



Some species of Grammoceras — as, for instance, Gramm. aalense — possess ribs 

 which become more or less united on the inner area, and this gives them the 

 appearance of Ludwigia. But the ribbing differs, in reality, essentially from that 

 of Ludwigia, because, not only is this junction of ribs so irregular — no well- 

 marked primary ribs, extending half across the side, being found as in Ludtvigia 

 — but the ribs on the outer area are not placed at an angle to those on the 

 inner area. Again, the forward ventral sweep is much more pronounced in 

 Grammoceras ; and the suture-line is less developed. In Plate A figs. 8 and 45 will 

 show the difference in the ribbing of Ludivigia and Grammoceras. 



What may be considered as a very high-developed form of Grammoceras, that 

 is, a species which has made great advance in the matter of involution, is Gram- 

 moceras suhserrodens, which converges in shape to Lioceras opalinum most remark- 

 ably, and illustrates the tendency of the species of the different branches of the 

 family Hildoceratidse to pass, sooner or later, through the same changes, and so 

 become similar in shape. Fortunately, we have in this case the fact of the suture- 

 line to guide us, and we can see (PL XXXI, figs. 9, 12, 14) that the simple 

 suture-line of Gramm. suhserrodens differs from that of Lioceras opalinum (Fig. 1, 



what he has found to obtain among other families. Among his papers on the subject, the following 

 especially may be consulted with advantage, and will show how much more deeply his investigations 

 have penetrated. 



1874. " Genetic Eelations of the Angulatidce," ' Proc. Boston Soc. Natural History,' vol. xvii, 

 pp. 15-33. 



1874. "Biological Eelations of the Jurassic Ammonites," ibid., vol. xvii, pp. 236 — 241. 



1876. " Genetic Eelations of Stephanoceras," ibid., vol. xviii, pp. 380 — 401. 



1883. " Genera of Fossil Cephalopoda," ibid., vol. xxii, pp. 253—338. 



1884. " The Evolution of the Cephalopoda," ' Science,' vol. iii, Nos. 52, 53. 



^ "Beitrage Monogr. Harpoceras," 'Neues Jarbuch fiir Mineralogie,' &c., Beil.-Bd. iii, p. 638. 

 2 It may be better to explain that I use this word in reference to those species which have passed 

 through the most changes, even though the later changes may appear to indicate decadence. 



