CEPHAIOPODA. 183 



nautilus {N. arietis, Eeinecke, = N. bidorsatus, ScUotlieiin), ia 

 found, and two kinds of rhynchoUte ; one sort, corresponding 

 with the upper mandible of the recent nautilus, has been called 

 " rhyncholites hirundo" (PI. II., Fig. 11); the other, which 

 appears to be only the lower mandible of the same species, has 

 been described under the name of " conchorhynchus avirostris."* 

 They also occur in the belemnite beds of the middle lias of 

 Dorsetshire ; these latter are very different in form from those 

 of nautili in the lower lias, and may probably belong to 

 belemnites. 



In studying the fossil tetrahrancJiiata, it is necessary to take 

 into consideration the varying circumstances under which they 

 have been preserved. In some straf a (as the lias of Watchett) 

 the outer layer of the shell has disappeared, whilst the inner 

 nacreous layer is preserved. Mo?.'e frequently only the outer 

 layer remains ; and in the chalk formation the whole shell has 

 perished. In the calcareous grit of Berkshire and Wiltshire the 

 ammonites have lost their shells; but perfect casts of the 

 chambers, formed of calcareous spar, remain. | 



Fossil orthocerata and ammonites are evidently in many 

 instances dead shells, being overgrown with corals, serpulae, or 

 oysters ; every cabinet affords such examples. In others the 

 animal has apparently occupied its shell, and prevented the 

 ingress of mud, which has hardened all around it ; after this it 

 has decomposed, and contributed to fonn those phosphates and 

 sulphides commonly present in the body- chamber of fossil shells, 

 and by which the sediment around them is so often formed into 

 a hard concretion. | In this state they are permeated by mineral 

 water, which slowly deposits calcareous spar, in crystals, on 

 their walls ; or by acidulous water, which removes every trace 

 of the shell, leaving a cavity, which at some future time may 

 again become filled with spar, having the form of the shell but 

 not its structure. In some sections of orthocerata it is evident 

 that the mud has gained access to the air-cells ; but the cham- 

 bers are not entirely filled, because their lining membrane has 

 contracted, leaving a space between itself and certain portions 

 of the walls, which correspond in each chamber. 



The tetrabranchs could undoubtedly swim, by their respira- 

 tory jets ; but the discoidal nautili and ammonites are not well 



* Lepas avirostris ( Schlotheim'), described by Blainville as the beak of a brachiopod! 



t Called spondylolites by old -writers. 



X In he alum-shale of "Whitby innumerable concretions are found, which, -when 

 struck with the hammer, split open and disclose an ammonite. See Dr. Mantell's 

 " Thoughts on a Pebble," p. 21. 



