GENERA OF AMMONITID.E. 



81 



Fig. 470. — Calcareous and 

 divided Aptychus (Trigond- 

 litcs lamcllosus) of an Am- 

 monite. Jurassic. (After 

 Woodward.) 



tlie aperture of the Ammonites, or wliich may occur in de- 

 posits wliicli contain Ammonites. When the Aptychus is cal- 

 careous, it consists of two shelly plates, divided by a median 

 suture (fig. 470), smooth or variously sculptured on one side, 

 and marked internally by concentric 

 lines of growth. AptycM of this nature 

 have been described as distinct fossils 

 under the name of Tngoiiellites. In 

 other cases, the Aptychus is of a cor- 

 neous consistence, and it is then in the 

 form of an undivided plate. Aptychi 

 of this kind are known by the general 

 name of Anaptychus. What may be the 

 precise nature of these extraordinary 

 structures has been a matter of much 

 dispute. They have been supposed to 

 be Crustaceous and allied to the recent 

 Barnacles ; but there can no longer be any doubt as to 

 their truly belonging to the Ammonites, with which they 

 are associated. Admitting this connection, the two principal 

 theories as to the nature and functions of the Apityclms 

 would regard this structure either as being an op>cr- 

 culuiii, similar to that of the Gasteropods, or as forming 

 a protective plate to the nidamental gland. The general 

 view, perhaps, is that it is an operculum, though there are 

 considerable difficulties in the way of an unconditional 

 acceptance of this opinion. Without entering further into 

 these, we must not omit to notice the important fact that 

 the theory that the AptycJms is a protective plate developed 

 in the walls of the nidamental gland, of necessity carries 

 with it the conclusion that all examples of Ammonites pos- 

 sessing this structure must be females. We must also not 

 forget that many Ammonites have never been shown to be 

 accompanied by any structures of the nature of the Aptychus, 

 and may therefore fairly be judged not to have possessed 

 one. 



It would occupy too much space here to enter at any 

 length into the subdivisions of the genus Ammonites which 

 are based upon the considerations above mentioned, and 



VOL. II. F 



