A New Ammonite Fauna of the Lower Turanian of Mexico 201 



following species : M. Swallowi Shumard, M . gibbosum Hyatt, M. Whitei 

 Hyatt, M. acceleratum Hyatt, and M. Kanabense Hyatt (figured but not de- 

 scribed). These are forms similar to Acanthoceras in one direction and to 

 Pulchellia in the other ; they are always involute, with relatively flat flanks, 

 a nearly rectangular or lanceolate cross section, much higher than broad. 

 The ornamentation consists of straight or slightly flexous ribs which begin 

 at the umbilical border without forming real nodules in that region, al- 

 though in some cases they show slight thickening in that place. The ribs 

 continue on the flanks, showing near the ventral border a thickening or 

 very little pronounced nodule, and on the ventral shoulder a very sharp nod- 

 ule which is somewhat longitudinally prolonged and very near the above 

 mentioned thickening. Between the main ribs other secondary ribs are 

 intercalated which begin above the umbilical border in different heights ; 

 the main ribs are often bifurcated. Between the two rows of nodules on 

 the ventral shoulders there is a flattened zone, smooth in some cases while 

 in others the very low ribs cross this zone. The nodules of the two ventral 

 shoulders frequently form a kind of undulated or interrupted keels. 



The suture of small individuals resembles that of Heinzia and Pulchellia, 

 while that of larger specimens is similar to the suture of certain groups of 

 Acanthoceras (Mantelliceras) and Acompsoceras. Characteristic is the ex- 

 ternal saddle divided by a secondary rather deep lobe ; the first lateral lobe 

 is broad and not very deep, the first lateral saddle is narrow but in general 

 higher than the external saddle; the second lateral lobe is narrow and 

 short, showing scarcely half of the depth of the first one. The second lat- 

 eral saddle is similar to the first one but much lower; there are always 

 three or four auxiliary saddles or even more in very large specimens. All 

 the saddles are very little ramified, the auxiliary ones often entirely whole. 

 The first lateral lobe seems to be always bifid. 



Very characteristic also is the manner of involution in this genus. While 

 the internal whorls are completely involute, the external whorl covering 

 the greater part of the inner one, we see that the larger whorls and the 

 animal chamber show a much wider curve and cover a much larger part of 

 the next smaller whorl. 



The species belonging to this group are so characteristic that one has to 

 consider them as an independent genus or subgenus. Hyatt considers this 

 group as a special family, giving it a position between the Heinzidae and 

 the Mantelliceratidae. 



To the species united by Hyatt in the genus Metoecoceras we probably 

 have to add some European forms. One of these is Pulchellia Gesliana 

 Petrascheck 1 from the upper Cenomanian of Saxony. Petrascheck con- 



1 Petrascheck, Amm. d. sachs. Kreideform., p. 140, pi. 7, figs. 3-5. 



