CEPHALOPODA. 81 



internal structure to the cancellated tissue of bones. Their external surface is 

 smooth or sculptured ; the inner side is marked by lines of growth. Forty- 

 five kinds are enumerated by Bronn ; they occur in all the strata in which 

 ammonites are found, and a single specimen has been figured by M. D'Archiac, 

 from the Devonian rocks of the Eifel, where it was associated with goniatites* 



Calcarious mandibles or rhyncholites (F. Biguet) have been obtained from 

 all the strata in which nautili occur ; and from their rarity, their large size 

 and close resemblance to the mandibles of the recent nautilus, it is probable 

 that they belonged only to that genus.f In the Muschelkalk of Bavaria one 

 nautilus (N. arietis, JReinecke, = N. bidorsatus, Schlotheim,) is found, and two 

 kinds of rhyncholite ; 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."^: 



In studying the fossil tetrabranchiata, it is necessary to take into consi- 

 deration the varying circumstances under which they have been preserved. 

 In some strata (as the lias of Watchett) the outer layer of the shell has dis- 

 appeared, whilst the inner nacreous layer is preserved. More frequently only 

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

 perished. ' In the calcarious grit of Berkshire and Wiltshire the ammonites 

 have lost their shells ; but perfect casts of the chambers, formed of calcarious 

 spar, remain. 



Fossil orthocerata and ammonites are evidently in many instances dead 

 shells, being overgrown with corals, serpulse, 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 il ; after this it has 

 decomposed, and -contributed to form those phosphates and sulphurets 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 



* The trlgonellites have been described by Meyer as bivalve shells, under the 

 generic name of aptychus; by Deslongchamps under the name of Munsteria. M. 

 D'Orbigny regards them as cirripedes ! M. Deshayes believes them to be gizzards of 

 the ammonites. M. Coquand compares them with teudopsis; an analogy evidently 

 suggested by some of the membranous and elongated forms, such as T. sanguinolarius, 

 found with am. deprcssus, in the lias of Boll. Ruppell, Voltz, Quenstedt, and Zieten, 

 regard the trigonellites as the opercula of ammonites, an opinion also entertained by 

 many of the most experienced fossil collectors in England. 



t M. D'Orbigny has manufactured two genera of calamari.es out of these nautilus 

 beaks! (rhynchoteufhis and. paltzoteuthis). In the innumerable sections of ammonite& 

 which have been made, no traces of the mandibles have ever been discovered. 



1 Lepas avirostris (Schlotheim), described by Blainville as the beak of a 

 brachiopod ! 



Called spnndylolites by old writers. 



|| In the 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. 



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