42 FAUNA OF THE CORNBRASH. 



Macrocephalites typicus, nora. nov. Plate III, figs. 1, 2, 3 ; Plate IV, fig. 5. 



1846. Ammonites macrocephalus, D'Orbigny, var. comprime'e, Pal. Franc., Terr. Jm\, pi. cli, p. 430. 

 1881. Stephanoceras macrocephalum, Nikitin, Mem. Soc. Moscow, vol. xiv, pi. iii, figs. 15-17. 



1884. Macrocephalites macrocephalus, Zittel, Handb. Palseont., p. 470, fig. 655. 



1885. Nikitin, Mem. Soc. Moscow, vol. xv, pi. (viii) x, fig. 44. 



Type (PL III, fig. 1). — Sides compressed, umbilical wall quite distinct from and 

 vertical to the sides ; diameter of umbilicus about ^ of the whole diameter of the 

 shell. Ribs scarcely touching the umbilical wall, but commencing on the sides 

 with a backward bend, where they are strong and few, forming sigmoidal curves, 

 with the concavity forwards, breaking up into and intercalated with 3 or 4 times 

 as many ribs, which gradually expand so as to be close-set on the periphery, and 

 assume ultimately a radial or slightly forward direction. The ribs on the cast do 

 not accurately coincide in position with those on the shell, so that the thickest part 

 of the shell is a little behind the rib-cast and the thinnest part a little in front, till 

 towards the aperture the surface ribs occupy the furrows on the cast. No sutures 

 are seen on this specimen, so the body chamber occupies at least § of the last 

 whorl, and the thickness decreases relatively towards the aperture. Numerical 

 details are given in No. 35 of Table I. From Scarborough, in the Sedgwick 

 Museum. 



Description. — The smallest specimen, also from Scarborough (No. 1 of Table I) 

 shows no alteration of characters beyond the fact that the ribs do not combine 

 into bundles towards the inside, while the umbilical edge is rounded. One from 

 Peterborough (No. 27) shows the last suture followed by a complete whorl, and a 

 third in the Sharp Collection at Birmingham shows the same form of sutures on 

 a larger scale. The ratio of the outer to the inner ribs is mostly over 3, and they 

 show a well-marked forward sweep. The breadth of whorl is mostly over 50 of 

 the corresponding diameter, except in one example from Thrapston doubtfully 

 placed here, but the thickness of the whorl, except in one case from Fairford, is 

 always a good deal less than its breadth ; in fact, the species is specially dis- 

 tinguished by its thinness, as may be seen in D'Orbigny's figure. But he 

 distinguishes two varieties among the specimens he refers to the species. The 

 present he calls the compressed variety, and the other may be the species described 

 by Schlotheim, as he states the two differential characters of the latter to be (1) 

 its compression, (2) the greater number of the ribs. It seems, however, that at 

 a later date he adopted Reinecke's name of tumidus for this, since he says it differs 

 from A. macrocephalus by its more spherical form and fewer ribs, though at the 

 same time he gives the thickness of the last whorl as - 81 only, against '87 for the 

 swollen variety of macrocephalus. 



