1913 TRACHYLYTOCERAS vii 



Nov. 



The explanation of this genus would appear to be thus : — It has 

 a Liparoceratid origin, starting from a stock similar to Beaniceras, from a 

 tuberculate cadicone, and it develops its feature of carination, repeating 

 the distinction which parted the Echioceratidae from the primitive 

 Liparoceratidae. It suggests that Paltopleuroceras, and hence the 

 Amaltheidae may be other carinate developments of Liparoceratid 

 origin — that the coronetted cadicone of their ontogeny (Buckman, 

 p.J.G.S. 1889, xlv, 652) indicates an ancestor similar to Beaniceras 

 centaurus, d'Orbigny sp. 



If the Amaltheidae are another stock which, like the Echioceratidae, 

 part from Liparoceratids by development of carination, then Defossiceras, 

 though Liparoceratoid in early whorls, should, from its carination, be 

 included in the Amaltheidae as an ally of, but more primitive than, 

 Paltopleuroceras. 



Genus, MERCATICERAS, nov. 

 Type, Ammonites mercati, Hauer, 1856, xxin, 6, 7 



Like Hildoceras, Hyatt, in form, with simple, broad lobed suture- 

 line, but distinct because radial curve has much less lateral flexure, 

 and there is no lateral furrow. Like the subflexiradiate series of Pseudo- 

 grammoceras, S. Buckman, in radial curve, but distinct because of simpler 

 suture-line, and more pronounced furrows on the venter. 



To Pcecilomorphus, S. Buckman, there is some likeness ; but in that 

 genus the radial line is more flexed laterally, ribs are often conjugate, 

 the furrows on venter are smaller, and the suture-line is somewhat more 

 developed. There is, too, an important difference in morphic develop- 

 ment : Mercaticeras is passing from involute to evolute, PcecilomorpJnis 

 is reversing the process. 



Hauer, 1856, xxin, 4 — 9, figured three species : of these the form 

 in 6, 7, may be selected as species and genotype ; that in 8, 9, may be 

 styled M. involutum, that in 4, 5, M. umbilicatum. 



Renz, 1905, x, 2, figures as Hildoceras mercati, var. hellenica, a form 

 which is very distinct on account of its heavy ribbing. 



Genus, AMAUROCERAS, nov. 



Type, Am. ferrugineus, Simpson 



Like Amaltheus, but without a crenulate carina ; in fact, barely 

 carinate, the venter being merely sharpened. It is feebly rostrate, and 

 shows only fine lineate ornament. Suture-line not unlike that of 

 Amaltheus. Am. lenticularis, Young & Bird, Y.T.A. 20, ought perhaps 

 to be placed here. The genus may be a degenerate series of the 

 Amaltheidae. Derivation, from nuavpos, dark. 



Genus, TRACHYLYTOCERAS, nov. 



Type, Am. nitidus, Young & Bird, No. 86. 



Small Lytoceratids with small, plain, rursiradiate rib-like rings 

 Such rings are presumably developments of hair-like lines, and in 

 catagenesis a return to the same lineate ornament and then to smooth- 

 ness is to be expected. 



