20 FOSSIL MOLLUSCA OF THE CHALK. 



dorso truncato, laterihus subcarinato, medio planimculo ; aperturd sagittatd ; anfractibus 

 interioribus occuUis ; umbilico clauso. 



Shell discoidal, much compressed, partially smooth, but marked with very numerous 

 striae, which are only faintly visible on the middle of the whorl, where they curve away 

 from the mouth, and more distinct towards the margin, where they are nearly straight; 

 each of the strige is produced into a point projecting beyond the edge of the truncated 

 back, thus forming a shght keel on each side of the back ; whorls increasing very rapidly 

 in height, and entirely covering the umbilicus ; mouth sagittate ; lobes of the septa 

 numerous, and much subdivided. 



Diameter 8 inches, height of the outer whorl 5 inches, thickness l^^ths of an inch. 



Found in the Grey Chalk of Hamsey, near Lewes. The species is very rare ; and I 

 have only seen the two specimens figured, which were collected by Dr. Mantell, and are 

 now in the British Museum ; M. D'Orbigny states that it is very rare in France, and has 

 been found at Rouen and Cassis, in the beds of the same age as in England. 



This species and the next described, A. obtedus, are nearly related, and are remarkable 

 for the rapid increase of the height of their whorls, which entirely close the umbilicus ; and 

 also for the great flexure of the upper edges of the lobes of the septa, which project 

 forward in the middle of the whorls, in a very unusual degree. 



2. Ammonites obtectus, Sharpe. Plate VH, fig. 4. 



A. testa discoided, compressissimd ; lateribus delicatim costatis : dorso truncato, tri- 

 carinato ; costis mmierosissimis, complanatis, medio angulatis et tuberculatis ; carinis 

 dorsalibus nodulosis ; aperturd truncato -sagittatd ; anfractibus interioribus occultis ; 

 umbilico clauso. 



Shell discoidal, much compressed, with slightly convex sides, and a narrow truncated 

 back ; ornamented on the sides with numerous flat ribs, which rising at the umbilicus 

 slope forward in a straight line to the middle of the whorl, whence they bend back at a 

 sudden angle, and continue in a slight curve to the dorsal margin, where they terminate ; 

 every second or third rib bears a rounded tubercle at the angle, on the middle of the 

 whorl ; and here and there a rib is inserted between the others reaching only from the row 

 of tubercles to the dorsal margin ; the angle of flexure of the ribs varies with age, being 

 at first very obtuse, and changing gradually till it becomes about a right angle ; back 

 narrow and truncated, with three keels, which are ornamented with small knobs equal in 

 number to the ribs ; each lateral keel is marked ofi" from the side of the whorl by an im- 

 pressed line running very near the edge ; whorls increasing very rapidly in height, and 

 entirely covering the umbilicus ; section of the whorls sagittate with the point cut off" ; 

 septa with long, very narrow, and ramified lobes, which are only partially seen in the 

 specimen figured. 



