230 Mr. II. Sceley on Ammonites 



nearing the back arc curved mouthwards. In the umbilicus 

 arise four or five more or less deep, wide sulcations, which are 

 mostly fiexuous, being first directed a little forward, then 

 perpendicularly upward, and finally curved forward on the back 

 to the siphuncle, the two sides meeting there in a broad V-shape 

 and dying away without impressing it. Between each two sulci 

 there arc commonly from twelve to eighteen ribs. 



The septa consist of the dorsal lobe, three lobes on each side, 

 and one or two in the umbilicus, all except the last much 

 branched, digitated, and close Jtogether. The dorsal lobe is 

 longer than wide, has three notches on each side, and terminates 

 in two large branches, which are cleft laterally, the parts which 

 continue backward being also cleft. The superior lateral lobe 

 has two or three small notches on each side, and terminates in 

 three large branches, which are trifurcate and digitated. The 

 dorsal saddle is larger than the lobe; it is centrally cleft, and 

 has a few notches, besides each half being cleft. The inferior 

 lateral saddle is much smaller, but similar. 



This form, which differs a little from those most common at 

 Cambridge, has been described first, because one of the types of 

 it is the identical specimen figured by Mr. Daniel Sharpe as A. 

 planulatus, and, moreover, it is the most inflated modification 

 that occurs. The example figured, Pakeont. Monogr. Cret. Mol. 

 pi. 12. fig. 4, had septa to the end ; and so has one of 4 inches 

 diameter, the largest I have; but fragments with the shell pre- 

 served occur, showing the back of a whorl to have been from 4 

 to 5 inches wide. The small line-like ribs passing over it are in 

 this large size a quarter of an inch wide, though but little ele- 

 vated, being separated by narrow shallow grooves. The sulca- 

 tions go on deepening and widening. 



On comparing small examples with corresponding specimens 

 of A. Mayorianus, D'Orb., from the Upper Greensand of the 

 Perte du Rhone, I fail to notice any distinction : compare them 

 as we may, there is no character they have not in common ; 

 consequently no hesitation is felt in regarding our form as iden- 

 tical with those of France and Switzerland; but it is not so 

 absolutely identical with the Chalk fossil known as A. planu- 

 latus. I know that form from a beautifully distinct cast of the 

 umbilicus partly formed by an Exogyra growing attached to the 

 specimen ; it is from the lower Chalk of Burwell in Cambridge- 

 shire. The umbilicus measured not less than 4 inches across, 

 and was shallow. The umbilical part of the whorls is flat, and 

 the depth of the ventrum exposed only one-fourth the width of 

 the uuembraced part of the side. On the Exogyra are the 

 foliations of the septa. And this enables me to state that the 

 sulci, which were straight on the sides, were not a sixth of the 



