from the Cambridge Greensand. 233 



so, confusion may be avoided by marking Cambridge specimens 

 A. navicularis, var. nothus. 



Diam. 2^ inches, with septa to the end. 



Ammonites rhamnonotus. PI. XI. fig. 7. 



Few-whorled, flat, with a round back and small umbilicus. 



Ornamented with about thirty-six radiating ribs, which con- 

 tinue uninterruptedly over the back, and are alternately long 

 and short. The long ribs mostly arise in the umbilicus, and the 

 shorter ones at a third or half the width of the whorl from the 

 back; they are nearly straight, elevated, and become tumid 

 where the side rounds into the back, but are most elevated in 

 the middle of the side. On the back the ribs are rather less 

 distinct, bend slightly towards the mouth, and each bears in its 

 centre a small sharp tubercle. In a younger state there are 

 also tubercles at the extreme edge of the back, which seem to 

 disappear with a diameter of twelve lines. These are, moreover, 

 the only tubercles in specimens of five lines diam., the back being 

 till then smooth and rounding. 



Mouth twice as high as wide, forming more than half the 

 diameter of the shell. # 



Septa complicated, divided on each side into four lobes. The 

 dorsal, which is wider and shorter than the superior lateral, is 

 ornamented with two branches on each side ; the lower of these 

 has three digits. The saddles are all half as wide again as the 

 lobes they correspond to, and divided by an accessory lobe into 

 two unequal parts. The superior lateral lobe, which is long and 

 narrow, has on each side two branches, of which the lower has 

 three digits, and in the middle a branch which bifurcates. In 

 the inferior lateral lobe the terminal branch does not bifurcate, 

 but terminates in three digits. 



Height of shell If inch ; height of umbilicus less than £ inch. 

 Height of mouth 1 inch, width \ inch. 



It nearly resembles A. sexangulatus \ but the much more 

 numerous and finer ribs, differently arranged, and the absence 

 of lateral tubercles from the back, readily distinguish it. The 

 A. Itierianus, D'Orb., has a very distant resemblance. Essen- 

 tially the shell is a compressed form of A. Mantelli, with a me- 

 sial row of dorsal tubercles instead of two lateral rows. 



Loc. Cambridge. Coll. University Museum. 



Ammonites sexangulatus. PI. XI. fig. 1. 



Few-whorled, discoidal, compressed, with an angular tuber- 

 culated back and small umbilicus. 



Ornamented with about twenty-five wide, rounded, radiating 

 ribs, which are somewhat wavy, being for the most part bowed 



