182 



MANUAL OF THE MOLLTJSCA. 



served for the passage of the funnel, whilst the upper 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 ammonites we have evidence that the aperture 

 was guarded still more effectively by a horny or shelly opeixidum, 

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

 {arietes), the operculum consists of a single 

 l)iece, and is horny and flexible.* In the 

 round-hacked 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 trigonellites, and pointed out the re- 

 semblance of their internal structure to the 

 _ cancellated tissue of bones. Their external 

 •^^^' ^^""^ surface is smooth or sculptured; the inner 



side is marked by linos 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 figiu-ed byM. D'Archiac, 

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

 with goniatit€S.\ 



Calcareous mandibles, or rhyncJioUtes (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 



* Tliis form was discovered by tlie late Miss Mary Arming, the indefatigable collector 

 of the lias fossils of Ljnne Regis, and described by INIr. Strickland, Geol. Journal, vol. i., 

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



t Trigonellites laviellosus. Park. Oxford clay, Solenhofen (and Cliippenham), 

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

 specimen in the cabinet of Charles Stckes, Esq. 



% The trigonellites have been described by Meyer as bivalve shells, mider the generic 

 name of aptyclius ; by Deslongchamps under the name of Munsteria. M. D'Orbigny 

 regards them as cuTipedes ! M. Deshaj^es 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. sanguinolariusj 

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

 regard the trigonellites as the opercida of ammonites, aw opinion also entertained by 

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

 described by Eolle (1862) as Cyclidia and Scaphanidia. 



§ M. D'Orbigny has manufactured two genera of calamaries out of these nautilus 

 beaks {rhynchoieidhis and 2^ai(^otei(this). In the innumerable sections of ammonites 

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



