21 



of little extent (from three to four lines), and is dilated and flattened above so 

 as to form a triangular surface half an inch broad at the base. In the lower 

 mandible the proportions of the two laminae are reversed, the exterior one 

 {d. fig. 2 ; b. fig. 4. pi. 8.) being produced to the fall extent, so as to make it 

 appear larger than the upper mandible, which is not really the case. 



The calcareous extremities of botli mandibles are of a hardness apparently ade- 

 quate to break through the densest crustaceous coverings, or even shells of mode- 

 rate tliickness. That of the upper mandible is sharp-pointed, and solid to the ex- 

 tent of five fines from the extremity ; but in the lower one the calcareous matter is 

 deposited on both sides of a thin layer of the black horny substance (a. fig. 4. pi. 8.), 

 and thus a combination of tough with dense matter is obtained, which much 

 dinfinishes the fiability to fracture. This mandible is also more hooked than 

 the upper one, but is more obtuse at the end : it seems from its dentated margin 

 evidently intended to break through hard substances, whilst the sharp edges of 

 the beak of the Cuttle-fish better adapt it for cutting and lacerating the soft 

 bodies of fish. Indeed in the particulars just mentioned, the mandibles of Nau- 

 tilus differ from those of every other known species of recent Cephalopoda. There 

 are, however, certain fossils called Rhyncolites, formerly considered to be the 

 beaks of fossil birds, but recognised by Blumenbach as appertaining rather to 

 the Cephalopoda, although evidently differing from all the recent genera then 

 known. jM. d'Orbigny ha\'ing invariably met with a large kind of these Rhyn- 

 colites in the same stratum wth the fossil shell of a large Nautilus {Nautilus 

 Gigas), suspected from that circumstance that they might be the mandibles of 

 that species. (See his Memoir in the Annales des Sciences Nat. v. p. 211. pi. 6.) 

 The calcareous extremities of the mandibles of Nautilus Pompilius, and the pe- 

 culiarities of their form, especially the flattened superior surface of the upper 

 mandible, fully confirm that conjecture, and at the same time show that a small 

 portion only of the beak is represented by the fossil. The transversely striated 

 body, which is figiued with the lower mandible at b. fig. 1. and a. b. fig. 2. in 

 the plate above quoted, is the part which supports the tongue of the Nautilus, 

 and which may be considered to represent an os hyoides. 



In the fleshy basis {d. fig. 5. pi. 8.) upon which the mandibles are encased, I 

 observed no other pecuharity than a distinct fascicidus of fibres, which, arising 

 on each side from the posterior margin of the upper mandible, runs along the 



