AMMONITES — ZOOLOGICAL, ETC. 



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a single median dorsal tongue prolonging itself on the back 

 considerably beyond the last ribs (see figure, PL 2. Fig. 2.) 

 in the form of a long beak. This disposition, which I have 

 observed in a large number of Species, is common to two 

 very different modifications of external form : 1st. to Ammo- 

 nites provided with a salient dorsal keel, as in all the species 

 of the Group Ai'ietes (in the Jurassic Rocks A. Turneri, 

 multicostatus, rotiformis, Bucklandi, obtusus, he.) ; all 

 those of the Group Crist ati (of the Cretaceous rocks. A, 

 cristatus, Bouchardianus, Roissyanus, varians); and some of 

 the Falciferi of the Jurassic formation ; 2nd. to Ammonites 

 provided with a deep canal in dependant of the ribs, charac- 

 terising the Group of the Tuherculati (of the Cretaceous 

 rocks, A. lautus, taberculatus). Thus Ammonites provided 

 with a dorsal keel or a deep canal, belong to the same divi- 

 sion of apertures, and are much more nearly allied by this 

 character, than one would have supposed by their external 

 form. 



In the second series of regular apertures, the margin not 

 only forms a tongue or dorsal beak, corresponding to the 

 mesial line of the back, and to the keel or canal j but there 

 is, moreover on each side of the shell, another tongue, 

 equally salient, corresponding to the angles or deep inflec- 

 tions which we observe on the ribs or lateral striae of the 

 shell. This form of margin is found in the two Groups which 

 I have already noticed. Amongst the Falciferi {Jl, bifrom 

 {Walcotii, Sowerby), serpentinus, ^c.) and the Tuberculati 

 {A. falcatus) ; it appears then to intimate the near alliance 

 of these two Groups. 



A third series of regular apertures, the most simple of all, 

 is that which appears to exist amongst a great number of 

 Ammonites, in which, hitherto, we have not recognised any 

 other margins than those of the lines of growth, more or less 



