CEPHALOPODA. 81 



internal structure to tlie cancellated tissue of bones. Their external siurface is 

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

 five kinds are enmnerated by Bronn ; they occur in aU the strata in which 

 ajumonites are found, and a single specimen has been ligm-ed by M. D'Arcliiac, 

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



Calcarious mandibles or rhyncholites (F. Biguet) have been obtained fi"om 

 aU the strata in which nautili occm' ; and from their raiity, their lai'ge 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 Bavai-ia one 

 nautilus [N. arietis, Keinecke, = N. bidorsatus, Schlotheim,) is found, and two 

 kinds of rhyncholite ; one sort, coiTesponding \\ith the upper mandible of the 

 recent nautilus, has been called " rhyncholites hirundo" (pi. II., fig. 11), the 

 other, which appeal's to be only the lower mandible of the same species, has 

 been described mider the name of " conchorh^Tichus 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 sheU has dis- 

 appeared, whilst the inner nacreous layer is preserved. ]\Iore frequently only 

 tlie outer layer remains ; and in the chalk formation the whole sheU lias 

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

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

 spai', remain. § 



Fossil orthocerata and ammonites are evidently in many instances dead 

 shells, being overgro\\'ii with corals, serpidse, or oysters ; eveiy 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 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 tr'tgonellites have been described by Meyer as bivalve shells, under the 

 generic name of aptyclius; by Deslongchamps under the name of Micnsteria. M. 

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

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

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

 found with am. deprfissus, in the lias of lioll. Ruppell, Voltz, Quenstedt, and Zieten, 

 regard the trigonellites as the o/;crc?iZa of ammoniUs, an opinion also entertained by 

 many of the most experienced fossil collectors in England. 



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

 beaks ! {rhtjnchotenihis and palccoieuthis). In the innumerable sections of am mo nites 

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



X Lepas avlrostris (Schlotheim), described by Blainville as the beak of a 

 brachiopod ! 



§ Called spondylolites by old writers. 



II 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. 



E 3 



