92 MANUAL OF THE MOLLUSCA. * 



of the mantle only ; and of an internal nacreous lining, deposited by the whole 



extent of its visceral sm-face. There is an ammonite in the British Museum, i 

 evidently broken and repaii-ed during the life of the animal,* which shews 

 that the sheU w^as deposited yVowi within. In some species of ammonites the 



collar of the mantle forms prominent spines on the shell, which are too deep ' 



for the visceral mantle to enter ; they are therefore partitioned off (as in A, i 

 M'inatus, Lias) from the body whirl and air cells, and not exhibited in casts. 



The baculites, and ammonites of the section cristati, acquire when adult 



a process projecting from the outer margin of their shell. Certain other ' 

 ammonites (the ornati, coronati, &c.) form two lateral processes before they 



cease to grow (pi. III., fig. 5). As these processes are often developed in : 



very small specimens, it has been supposed that they are formed repeatedly i 



in the life of the animal (at each periodic rest), and axe again removed when i 



growth recommences. These small specimens, however, may be only dwarfs, i 



In one ammonite, from the inferior oolite of Normandy, the ends of these l 



lateral processes meet, "forming an ai'ch over the aperture, and dividing ■• 

 it into two outlets, one corresponding with that above the hood of the nautilus, 

 which gives passage to the dorsal fold of the mantle ; the other with that ,; 



below the hood, whence issue the tentacles, mouth, and funnel ; such a modi- | 



fication, we may presume, could not take place before the termination of the ^ 



growth of the individual. "f (Owen.) ^ 



M. D'Orbigny has figm'ed several examples of deformed ammonites, in * 



which one side of the shell is scarcely developed, and the keel is consequently \ 



lateral. Such specimens probably indicate the partial atrophy of the branchiae ; 



on one side. In the British ^Museum there are deformed specimens of Am. , 



ohtitsus, amaltheus, and tuherculatus. \ 



Fig. 53.t 



* A serpentinns Schloth, U. Lias, Wellingboro. Rev. A. W. Griesbach. 



t This unique and abnormal specimen is in the cabinet of S. P. Pratt, Esq. 



J Fig. 53. Goniatites sphericus, Sby. Front and side views of a specimen from the 

 carb limestone of Derbyshire, in the cabinet of Mr. J. Teiinant; the body- chamber 

 and shell-wall have been removed artificially. 



