AMALTHEUS OXYNOTUS. 389 



Descrijjtio7i. — In the above measurements we observe that the relative proportion 

 varies chiefly as regards the thickness of the shell and the width of the umbilicus ; in 

 other respects, the specific characters are very well maintained. 



The shell is discoidal, and very much compressed ; the whorls are high and extremely 

 involute ; the outer whorl being half the diameter of the shell. The sides are convex, 

 the inner margin gently rounded, and the outer third bevelled away into a thin, sharp 

 cutting edge, which gives value to the name par excellence, oxynolus (J^Jc, sharp ; I'wroc, 

 back), as it is the sharpest Ammonite extant. The ribs are slight obhque folds, thirty to 

 forty in number, which ascend from the spiral suture two thirds up the side ; here the 

 shell begins to bevel off, and the ribs at this point make a sharp angular bend towards 

 the carina (fig. 192), and dividing into two or three costse as they incline forward to the 

 aperture (fig. 191). 



The umbilicus is more or less open in different specimens, being very narrow in 

 young, and becoming wider with increasing age, the amount of involution varying from 

 one fourth to one sixth of a whorl. 



The aperture is acutely lanceolate (fig. 192), the sides of the last whorl deeply embracing 

 the penultimate one as shown in fig. 5. 



The lobe-line (PI. XLVI, fig. 6) is very complicated from the close approximation of 

 the septa. The siphonal lobe is wide with two divergent branches, each having three 

 lateral digits on each side, and a ramose terminal tuft. The siphonal saddle is also wide 



\ 



X 



Fig. 191. — Amaltheus oxynolus, Qurnst. 



Fig. 192.— Front' 



and shallow, and ends in five deep lobe-like folioles. The principal lateral lobe is much 

 smaller than the siphonal, and consists of a central stem with short lateral branches, and 

 a trifid terminal one. The lateral saddle is narrow and deep, and ends in five folioles. 



The lateral lobe is short and oblique with five digitations around its margin. The 

 auxiliary saddles are wide and shallow, and the auxiliary lobes mere short digitations. 



