182 



MAirtJAL OF THE MOLLUSCA. 



Berved for the passage of the fannel, wHlst the tipper and larger 

 space (c c) was occupied by the neck ; the lobes probably indicate 

 the position of the external arms. 



The aperture of the pearly nautilus is closed by a disk or hood 

 (Fig. 50, h), formed by the union of the two dorsal arms, which 

 correspond to the shell-secreting, arms of the argonaut. 



In the extinct aTnmonites we have eyidence that the aperture 

 was guarded still more ejffectively by a horny or shelly operculum, 

 secreted, in all probability, by these dorsal arms. In one group 

 {arietes), the operculum consists of a single 

 piece, and is horny and flexible.* In the 

 round-backed ammonites the operculum is 

 shelly, and divided into two plates by a 

 straight median suture (Fig. 49). They were 

 described in 1811, by Parkinson, who called 

 them trigoneUites, and pointed out the re- 

 semblance of their internal structure to the 



cancellated tissue of bones. Their external 



Fig. 49. t surface is smooth or sculptured; the inner 



side is marked by lines of growth. Forty-five kinds are enume- 

 rated by Bronn ; they occur in all the strata in which ammonites 

 are found, and a single specimen has been figured byM. D'Archiac, 

 from the Devonian rocks of the Eifel, where it was associated 

 with goniatites.\ 



Calcareous 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 man- 

 dibles of the recent nautilus, it is probable that they belonged 

 only to that genus. § In the Muschelkalk of Bavaria one 



* This form was discovered by the late Miss Mary Anning, the indefatigable collector 

 of the lias fossils of Lyme Eegis, and described by Mr. Strickland, Geol. Journal, vol. i., 

 p. 232. Also by M. Voltz, Mem. de I'lnstitiit, 1837, p. 48. 



t Tricjonellites lamellosiis. Park. Oxford clay, Solenhofen (and Chippenham), 

 associated with ammonites lingulatus, Quenstedt. (= A. Brightii, Pratt). From a 

 specimen in the cabinet of Charles Stckes, Esq. 



X The triyonellites have been described by Meyer as bivalve shells, under the generic 

 name of aptychus ; by Deslongchamps under tlie 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 sug- 

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

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

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

 many of the most experienced fossil collectors in England. Some of them have been 

 described by Eolle (1862) as Cydidia and Scaphanidia. 



§ M, D'Orbigny has manufactured two genera of calamaries out of these nautilua 

 beaks (rhi/nchoteuthis and palceoteuthis). In the innnmerable sections of ammonites 

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



