AMMONITES — ZOOLOGICAL^, KTC. 



43 



ones to appear, or render the two sides of the shell very un- 

 equal. When these modifications deform the shell and 

 destroy its symmetry, it is easy to perceive that they arise 

 from an injury received by the animal ; deformed specimens 

 of A. serpentinus, Brogniartii, radiatus, and Hwnphreisia- 

 nus, etc., which I possess, furnish sufficient proof of this ; 

 but when these changes take place on the mesial line of the 

 back, and change the form of this part entirely, the shell 

 being to all appearance quite regular, we do not then possess 

 any means of distinguishing the malformation, and we run 

 considerable risk of making an useless extension of species, 

 unless we have sufficient courage to break the specimens to 

 assure ourselves whether the internal whorls are the same. The 

 most extraordinary example which I can cite of this latter 

 mode of change is an individual of A. interruptus (PI. 2. 

 Fig. 1 .), where a species having a dorsal canal and the ribs 

 terminating in the middle of the back and alternating on 

 each side of the mesial line, has become an Ammonite having 

 a round back, with the ribs passing entirely over from one 

 side to the other. Some doubts having been expressed on 

 this extraordinary anomaly, I broke the specimen, and found 

 that the internal whorl instead of having the ribs passing 

 over the back, as in the last whorl, was provided in the pre- 

 ceding whorl with the dorsal canal and the alternating ribs 

 common to A, interruptus. This exemplifies the need of 

 extreme caution ; it is therefore indispensable, when we find 

 in any formation, an Ammonite of anomalous form, to be 

 well assured that all the whorls have the characters proper to 

 each period of growth, and that this anomaly is not caused 

 by an accidental deformity. 



Varieties from sex. — All who have studied Ammonites 

 carefully have remarked that individuals having externally 

 the same ribs, spines, and distributions of ornaments, have 

 at equal diameters, sometimes a very tumid and at other 



