214 GEOLOGY. 



from, its chamber. On the former supj^osition a cartilage may 

 have attached the bevelled edges to the peristome, admitting free 

 action for closing or opening the apertnrc at the will of the 

 animal. The truncate thickened edges when closed, would lie 

 at right angles to the dorsal edge: such an appendage is not 

 unusual to Cephalopods of the present day — the mouth of the 

 Nautilus pompilius, a recent species, is closed by the connection of 

 its two dorsal arms. If it was employed for protection when the 

 animal was protruded, it was probably used as a defence of the 

 branchiaj which laid immediately beneath the mantle. The 

 Ammonite swam by forcing out the water used for respiration 

 from the branchial sac, which if torn, the animal could no longer 

 direct its movements; some such means to preserve it from 

 harm was therefore necessary. The opening and shutting of the 

 mantle for respiration, would be much facilitated by the division 

 of this appendage, and there could not be a more beautiful adap- 

 tation for the purpose: at the same time it offered no hindrance 

 to the withdrawal of the animal within its chamber. 



Another view has been taken by some naturalists, that it 

 formed a part of the internal organization of an invertebrate 

 animal, acting as the framework of a fleshy envelope; a develope- 

 ment from a lower to a higher type of animal life. That they 

 belong to some part of an ammonite, however, there is little 

 doubt. 



The repetition of the same beds by a series of faults at and in 

 the neighbourhood of Kimmeridge Bay, favour their examination 

 as they emerge from the shore, and I have every reason to be- 

 lieve the Trigonellites belonged to the Ammonites longispinus; 

 both of which abound in a narrow zone beneath the Kimmeridge 

 coal, and are invariably associated together. 



If these plates were attached to the animal, they would na- 

 turally fall out of the shell at death, which may account for 

 their being scarcely ever found in pairs, being invariably separated 

 from the ammonite. The Trigonellites arc not confined to the 

 Kimmeridge Clay Shales; they occur as low down ns the Lias. 

 Mr. Sharp figures six British species in his Monograph, from the 

 middle of the Upper Chalks of Norfolk, and three from the 

 Chalk formation of Meudon in France. Mr. Braun enumerates 

 forty five kinds, alleging that they occur in every stratum in 

 which Ammonites are found. 



J. C. MANSEL. 



