118 PALAEONTOLOGY OF THE UPPER MISSOURI. 



sented by the shells of these mollusks, their great numbers, and often elaborately 

 ornamented surface, and remember the large sizes to which they sometimes attained, 

 it is easy to understand that they must have constituted a marked and peculiar 

 feature of the molluscan fauna of the Jurassic and Cretaceous seas. 



It not unfrequently happens, where the substance of the shell is well preserved, 

 that in breaking specimens from the rocky matrix in which they are enveloped, 

 the outer porcellaneous layer exfoliates, leaving the elegantly sculptured surface of 

 the fossil covered with the brilliantly iridescent inner pearly layer, in which condi- 

 tion they form exceedingly beautiful cabinet specimens. It is necessary, however, 

 to remove this inner layer also, when we wish to study the complex internal struc- 

 ture of the shell, which furnishes important characters for the distinction of species 

 and sometimes of genera. 



The following are the genera we would at present include in this family, viz : 

 Baculina, Bctculites, Ptychoceras, Hamulina, Samites, Toxoceras, Crioceras, Ancylo- 

 ceras, 1 Scaphites, Ceratites, Ammonites, Anisoceras, Helicoceras, Heteroceras, and 

 Turrilites. It seems to be impossible, however, by a linear arrangement, to place 

 these groups so as always to bring together those most nearly allied. 



REMARKS ON THE SO-CALLED GENUS TRIGONELLITES, OF PARKINSON, 1811. 



Aptychus, METER, 1831. Jchthyosyayon, BOURDET, 1822. Munsteria, DESLOSOCHAMPS, 1835. 



A consideration of the family Ammonitidce would scarcely be complete without 

 some allusion to those curious bodies generally known by the names TrigoneUites, 

 Aptychiis, &c., so often found within, or associated with, the shells of the typical genus. 

 Few objects amongst all the relics of extinct life have been more puzzling to the 

 palaeontologist, or given rise to a greater diversity of opinions than these. Most of 

 the early palaeontologists regarded them as the shells of bivalve mollusks, as did 

 Parkinson, Deslongchamps, and some later investigators ; while others supposed 

 them to be the palatal bones of fishes. Others, again, maintained that they are the 

 internal osselets of some extinct cephalopod allied to Teudopsis ; and still others, 

 that they are an internal organ of Ammonites, analogous to that connected with 

 the digestive apparatus of Bulla and some other Gasteropoda. Burmeister sup- 

 posed them to be external supplementary shell pieces of Ammonites, designed for 

 the protection of the branchial sack when the animal was partly protruded from 

 the shell. More recently, D'Orbigny, Pictet, and some others have advocated, with 

 much ingenuity, an opinion first suggested by Scheuchzer, that they are the valves 

 of pedunculated Cirripedes allied to Anatifa. 



The impression, however, has for some time been gaining ground amongst pala> 

 ontologists that these bodies really are organs or appendages of* the Cephalopods, 

 with the shells of which they are so frequently found associated. And since Darwin 



1 It is possible the genus Ancyloceras may be synonymous with Crioceras, since the species for which 

 the latter genus was proposed have never, we believe, been found entire ; and it yet remains to be 

 clearly demonstrated that Ancyloceras was not founded upon perfect specimens of the same type. If 

 so, the name Crioceras will take precedence, because it was published in 1836, and Ancyloceras in 1842. 



