AMMONITES. 33 



This species resembles A. Velledce, Mich., in its general form and elegant thread-like 

 ribs, but is at once distinguished by its larger umbilicus. The specimen figured is the 

 only one which has come under my notice. 



19. Ammonites Rhotomagensis, Defiance. Plate XVI, fig. 1 — 4. 



Ammonites Rhotomagensis, Brongniart. Environs de Paris, t. vi, fig. 2. 



— — Sowerby. Min. Conch., t. 515. 



— — Bronn. Lethaea Geogn., t. xxxiii, fig. 1-3. 



— — D'Orbigny. Pal. Franc., t. 105 and 106. 



— Sussexiensis, Mantell. Geol. Sussex, t. xxi, fig. 10. (Not t. xx, fig. 2.) 



A. testa discoided, costatd, tuberculatd : anfractibus tumidis, qicadrato-rotundatis : costis 

 rectis, simplicibus, rohmdatis, aqualibus ; junioribus septem, adultis sex-tuberculatis .• dorso 

 juniore rotundato, tuberculis triserialibus ; adulto bituberculato, transversim costato, vet 

 medio piano : umbilico magno : septorum lobis lateralibus inaqualiter bifidis ? 



Variat anfractibus planiusculis, tubercidatis ; costis subobsoletis ; dorso piano. 



Shell discoidal ; whorls tumid, flattened at the back and sides, with nearly a square 

 section ; about one third concealed ; crossed by about twenty-four simple, straight, rounded 

 ribs, which when young bear seven tubercles, and when full grown six tubercles each ; the 

 tubercles are thus placed, — one row near the umbilicus, and two rows on each side of the 

 back, the seventh row on the middle of the back disappears in adult shells, leaving the 

 back either smooth and flat, or crossed by a continuation of the ribs • umbilicus large and 

 ill-defined, with sloping sides. The lateral lobes of the septa are unequally bifid? 



Diameter, 8 inches ; width, 3 J inches ; height of aperture, 3 inches. 



Very abundant in the Grey Chalk of all the southern counties, and in the Chloritic 

 Marl of the Isle of Wight, and Chalk with silicious grains of the West of England. 



The A. Rhotomagensis, as above defined, presents very constant characters ; from its 

 youngest stage, fig. 3, till it has attained about two thirds of its full size, the only change 

 is a gradual increase in the number of the ribs, but the form and number of the tubercles 

 continues the same ; this distinguishes it from A. Sussexiensis and A. Cunningtoni, 

 in both of which the dorsal are, in young shells, more numerous than the lateral tubercles. 

 In the adult A. Rhotomagensis the tubercles on the middle of the back disappear ; in 

 some specimens the ribs are continued prominently across the back, in others the back is 

 flat and nearly smooth. The size at which the change takes place varies very much. 

 Specimens are often found which, though less than three inches in diameter, have all the 

 characters of the adult shell, one of these is represented of the natural size in fig. 2 ; but 

 more frequently they reach nearly six inches in diameter before losing the dorsal tubercles, 

 as in the case of fig. 1. The superior lateral lobe of the septa is always bifid, with more 

 or less unequal terminal branches ; the termination of the next lateral lobe varies, some 



