23-^ Mr. H. Seeley on Ammonites 



a little forward on the middle of the side : they are separated 

 by spaces never narrower than the width of the ribs. Less than 

 half of the ribs reach the umbilicus, two commonly uniting in 

 a fork at about a third the width of the whorl from it, and a 

 single free one (which dies away at the same distance) sometimes 

 occurring between pairs of forks. 



. The sides, which are parallel, slope into the umbilicus, and 

 make a large angle with each half of the back, by tubercles 

 being developed on the ribs at the line where the back would 

 begin to round. The ribs are prolonged, somewhat widening 

 and curving forwards, to the centre of the back, where they ter- 

 minate in prominent tubercles. Thus the back is ornamented 

 with three rows of tubercles. 



The mouth is six-sided, with the sides opposite and parallel. 

 It is half as high again as wide, and nearly half the height of 

 the shell. 



The septa appear to consist of three lobes on each side. The 

 dorsal is wider and shorter than the superior lateral ; they both 

 have three small branches on each side, and at the end two 

 larger ones trifurcate. The dorsal saddle, which is about as 

 wide as the lobe, is cut into by two very small accessory lobes. 



In a young state (diameter | inch) the sides are perfectly 

 parallel ; only one or two of the ribs reach the umbilicus, and 

 all the others are much shorter than the short ones in the larger 

 specimen. 



Height 1| inch; width of umbilicus \ inch. Height of 

 mouth 4-i- inch, width -^ inch. 



Loc. Cambridge. Coll. University Museum. 



This form belongs to the small series with trituberculated 

 backs, typified by A. papalis. I am not familiar with any form 

 which closely resembles it. A. Itierianus, D'Orb., has some 

 likeness to the young form ; but the much more numerous ribs, 

 smaller umbilicus, &c., easily identify the shell described. A. 

 Brottianus is nearly related. 



Ammonites acanthonotus. PI. XI. fig. 5. 



Few-whorled, compressed, with the sides gently inflated; 

 back rounded, bearing a mesial row of spines; umbilicus as 

 high as the whorl at the opposite side of the mouth. 



The umbilicus is shallow, with the lower third of the side, 

 which it includes, gently bevelled down to the preceding whorl ; 

 it is marked with radiating ribs, each terminating under the 

 succeeding whorl in a small tubercle. 



On the side of a whorl there are about twelve tubercles, from 

 which its upper two-thirds inclines inwards, rounding gently on 

 nearing the back. From each eminence diverge two ribs (in 



