32 



has one more whorl visible externall}-. The volutions of A. Loganianus 

 are coiled in a very similar manner to those of A. buUafus, D'Orbiguy,''' 

 but the sculpture and shape of each are perfectly distinct. 



The Oolitic Macrocephali, as a whole, are said to possess a combination 

 of characters by which the}' can generally be distinguished from Ci'eta- 

 ceous Ammonites of the same group. In the Oolitic species the shell is 

 more globose, the whorls are very strongly involute, and, as Stoliczka 

 has pointed out, " the lateral ribs form a tubercle about the mitldle of the 

 sides, and then divide into tw^o or more ribs." In each of these respects, 

 A. Loganianus has more the aspect of an Oolitic than of a Cretaceous 

 species. 



The name proposed for this shell is nitended as a tribute, of respect 

 and aftection, to the memory of the late Sir W. E. Logan. 



Group IV. — Coronarii, Buck. 



Sub-genus Stephanoceras, Waagen (Paks. )^" Geognostich-PaLiiontologischeBei- 

 trage." Munchen, 18C9. Vol. II., page 248. ) 



Ammonites RicnARnsoNii. (N. Sp.) 



Plate V. Both figures. 



Shell thick, iiiflated; umbilicus wide and deep, conspicuousl}' coro- 

 nated round its inner margin b}' a row of distant, rounded tubercles. 



Volutions about six, veiy closely and tigbtly coiled, so that their width 

 is about two-thii'ds greater than their height, much raised at the sides, 

 widest and sub-angular neai- their middle. The amount of involution is 

 always slight, and decreases extei-iorlj'- ; the inner faces of the early 

 volutions, and the whole of the sides of the last but one being fully 

 exposed. In the outer whorl the periphery is ventricose and rounded ; 

 its curve is confluent with that of the outer half of the sides, which 

 swell up (almost concavely) so as to form a sub-angular ridge about 

 their middle, but nearer to the sutures. From the simimit of this ridge, 

 which forms the outer margin of the uml)ilicus, the Avhorls slope 

 abruptly and almost precipitously down to the sutures, so as to present 

 a nearly straight (though slightly convex) umbilical face. In the last 

 lialf turn the umbilical margin becomes more rounded, and the inner 

 face of the whorl is more oblique aad spreading. As the greater part of one 

 side of the only specimen collected is worn away, the exact width or thick- 

 ness of the shell cannot be ascertained, but it was probably more than,one- 



I'aleniitolonie FraiiQaise. Terrains J urassiques." \'i>l. n. Atlas, Plate CXLH., figs. ] and 2, 



