852 TETR ABRANCHIATE CEPHALOPODS. 



described as the cephalic bucklers of Phyllocaridan Crustaceans 

 (such as some of the forms referred to Cardiocaris and Spathiocaris) 

 are in reality the " Aptychi " of Goniatites. Many theories have 

 been held as to the nature of the " Aptychi " of the Ammonoids, 

 but there are only two views which can be regarded as at all prob- 

 able. On one of these views, the Aptychi are to be regarded as 

 protective plates developed in the walls of the nidamental gland, in 

 which case all examples of Ammonites possessing these structures 

 are necessarily females. The more probable theory, however, is 

 that put forward by Sir Richard Owen — viz., that the Aptychus is 

 formed by the deposition of horny matter or lime within a structure 

 corresponding to the " hood " of the existing Pearly Nautilus. On 

 this view the Aptychus would act as an operculum, when the animal 

 was withdrawn within the shell. In those Ammonoids which are 

 devoid of Aptychi it must be supposed that the " hood " remained 

 permanently uncalcified, as it does in the Pearly Nautilus. 



As regards their zoological affinities, the Ammonoids have been 

 generally regarded as belonging to the Tetrabranchiate division of 

 the Cephalopoda, in which case the shell, like that of the Pearly 

 Nautilus, must have been external. It has, however, been also 

 held that the Ammonoids are really referable to the Dibranchiate 

 division of the Cephalopods, in which case the shell, like that of 

 the existing Spirula, must have been internal. The grounds for 

 this view are chiefly derived from the mode of development of the 

 shell of the Ammonoids, which resembles that observed in Spirula 

 — the protoconch being in each case inflated and destitute of a 

 cicatrix, while a " prosiphon " is present. The " Aptychi " of the 

 Ammonoids have also been compared to the " nuchal cartilages " 

 of the Cuttle-fishes. Upon the whole, however, the balance of 

 evidence is in favour of the reference of the Ammonoids to the 

 Tetrabranchiata — the groups of the Clymeniidcz and Goniatitidce 

 affording a transition between the two great sections of the Am- 

 monoidea and Nautiloidea. 



As regards the general distribution in time of the Ammonoidea, 

 the most ancient types of the group belong to the genus Goniatites, 

 in the wide sense of this name, and are found in the highest Silurian 

 and the lowest Devonian deposits. The genus Clymenia, which, 

 like Goniatites, is comparatively simple in morphological type, is 

 also found in Devonian strata. The earliest forms of the " Am- 

 monites " are found in the Permo-Carboniferous rocks of India, but 

 in other regions the earliest representatives of this great section 

 of the Ammonoids appear in the Trias. In the Mesozoic period 

 the Ammonoids undergo a vast development, the principal and 

 most widely distributed type being that of the " Ammonites." 

 In the Cretaceous rocks, however, numerous new and varied 



