92 



MANUAL OP THE MOLLTJSCA. 



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

 extent of its visceral surface. There is an ammonite in the British Museum, 

 evidently broken and repaired during the life of the animal,* which shews 

 that the sheU was depositedy/■o»^ within. In some species of ammonites the 

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

 for tlie visceral mantle to enter ; they are therefore partitioned off (as in ^. 

 arm/itus. Lias) from the body whirl and air cells, and not exhibited in casts. 



The bacuhtes, 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 (pL III., fig. 5). As these processes are often developed in 

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

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

 ^•owth recommences. These small specimens, however, may be only dwarfs. 

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

 lateral processes meet, "forming an arch over the apertm-e, and dividing 

 it into two outlets, one corresponding ^\ith 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 

 gro'wth of the individual."t (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 branchife 

 on one side. In the British Musemn there are deformed specimens of Am. 

 oitusus, amaltheus, and tuherculatus. 



Fig. 53.J 



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



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



X Fig. 53. Goniatites sphericus, Sby. Front and side viev.-s of a specimen from the 

 earb limestone of Derbyshire, in the cabinet of Mr. J. Tennant; the body-chamber 

 and shell-wall have been removed artificially. 



