L. F. Spath — Notes on Ammonites. 29 



principal lobe ; depressed forms on the other hand externally and 

 internally. Again, as during growth the septal Surface increases at 

 the relatively quickest rate on the external side, differentiation 

 must begin here ". 



On examination of the three types of Ammonites chosen by 

 Swinnerton & Trueman, it is found that in Tragophylloceras 

 Loscombi 1 with wide lateral areas, the first lateral lobe and first 

 lateral saddle show the greatest differentiation ; and in the depressed 

 Spharoceras Brongniarti it is the external saddle. In the 

 Dactyliocerate suture-lines figured by the authors on p. 39 the 

 widest-ventered form (fig. 4) has the external saddle strongly 

 developed, and it has already been remarked that there is a tendency 

 to equalize the size of the saddles on the adoption of a more 

 cylindrical whorl. 



It should be pointed out, however, that there are what may be 

 called family peculiarities that modify the suture-elements in 

 certain cases. They are of value in tracing the affinities of 

 homceomorphs, such as the perfectly similar oxycones that appear at 

 so many horizons. The Triassic JEntomoceras denudation, Mojsisovics, 

 and the Cretaceous Garnieria e.g. had been put into the strictly 

 Lower Liassic genus Oxynoticeras by different authors. Although, 

 in the mechanical adjustment to a wider side, either by the 

 spreading-out of the lateral lobes and saddles or by the addition of 

 auxiliary or adventitious elements, 2 similar suture-lines may result 

 in different stocks, yet the modified shells can generally be referred 

 to their ancestral stock by means of some retained family 

 characteristic. 



Again, the genera Macroceplwlites, Cadoceras, Pachyceras, Torn- 

 quistes, and Evymnoceras, with wide ventral areas, all have a very 

 large external saddle. In Chamoussetia, a smooth and keeled 

 descendant of the Cadoceras stock, what may be considered the 

 natural adjustment of the suture-line to this type of shell is shown; 

 yet in the later keeled Quenstedticeras and Cardioceras the suture- 

 line at first still is more or less similar to that of the fat ancestral 

 forms. This may partly be retention of the family character or 

 hastening of the development of the keel; but it maybe assumed 

 that ornament and other mechanical expedients for the increase of 

 the solidity of the shell also influence the septal edge. This may 

 account for the changing width of the external saddle in 

 Macrocephalites and Tornquistes to which R. Douville 3 has drawn 

 attention. Tragophylloceras ibex, with strong ornament, has a 

 simplified suture-line as compared with the smooth Phyllocerates 4 of 



1 The specimen used by Swinnerton and Trueman for their series of septal 

 sections (fig. 13 on p. 46) shows the small terminal leaflets of the ibex group. 



2 Pictet (Traite de Paleont., p. 6"69) pointed out already in 1854 that the 

 inflated ' ' varieties " of a species often differed from the compressed ones in 

 the number of the accessory lobes, and that modification of the umbilicus 

 produced the same result. 



3 "Etude sur les Cardioceratides de Dives, etc.": Mem. No. 45, Soc. 

 Geol. France, Pal., i, 19, fasc. 2, p. 14. 



4 Additional work on the various features of the suture-line has demonstrated 

 to the writer the impossibility of basing the separation of genera which other 



