MACROCEPHALITES COMPRESSUS. 45 



probable, however, that several of the specimens in the south generally named 

 only Macrocephalites may represent this species of the genus. 



Macrocephalites compressus, Quenstedt. Plate IV, fig. 4; text-fig. I. 



1849. Ammonites macrocephalus compressus, Quenstedt, Cephalopoden, p. 184, pi. xv, fig. 1. 



1886. — — Quenstedt, Am. Schwab. Jura, p. 651, pi. lxxvi, figs. 14, 



15 (but not fig 4). 

 Non 1881. Stephanoceras compressum, Nikitin, Mem. Acad. Imp. S. Petersb. [vii], vol. 28, pt. 5. 



Type. — " The ribs are extraordinarily fine, the mouth is also so strongly com- 

 pressed that the sides and the back together have a parabolic outline. Kate of 



whorl growth nKm = 1*85; of thickness growth TTTeTn = l' ; >8." From the 



Macrocephalus beds of Lochen and Laufen. 



Description. — -This species is distinguished by the fineness and regularity of its 

 ribs, which run in a nearly, but not quite, radial direction, and do not combine 

 into groups towards the umbilicus, but tend to die away altogether. The sides are 

 tolerably flat, but the greatest thickness is near the umbilicus. This would seem 

 to make it a very distinct species, nevertheless when it is young it is hard to 

 distinguish from M. tt/picus, and it is not therefore very certain in its range. The 

 specimen figured- (PI. IV, fig. 4; B.M. 8321) was obtained from the interior of a 

 large dogger, the remainder being the outer whorl of the ammonite, which shows 

 sutures on the outside. No measures are here possible except the ratio of the 

 thickness, which is '52 of the corresponding diameter. The ribs number 48 in a 

 half-whorl, and slant somewhat forwards, and cross the edge of the umbilicus, which 

 is vertical. On the outside of the large shell some remains of ribs arc also seen. 



From the same pit is another example — the largest seen of any Macrocephalites 

 (No. 37 of Table I, B.M. 8322). It is 14 inches in diameter, and has 22 ribs left on 

 less than one fourth of the circumference, and the remainder is body chamber, on 

 which the ribs are first replaced by wide and obscure undulations and finally by 

 smoothness. In this condition it might well be called terebratuSybvA it is not worth 

 while, since this is the final condition of all the large shells. For the same reason 

 it has a tumid aspect, though it is really one of the thinnest only "42 of its 

 diameter. Its umbilicus is vertical, and very complicated and crowded sutures 

 are seen beyond the body chamber, as shown in text-fig. 4. 



By these and the other characters may be recognised smaller specimens, 

 especially when, as at Castor, they lie side by side with .1/. typicus. Waagen, in his 

 ' Cephalopods of Cutch,' supposes the present type to be characteristic of the true 

 Macrocephalus, from which we may conclude that it is common in Germany, where 

 Quenstedt names it. It is, however, a special form when adult, though the young 



