182 MANUAL OF THE MOLLUSCA. 
served for the passage of the funnel, whilst the upper and larger 
space (cc) 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, 4), formed by the union of the two dorsal arms, which 
correspond to the shell-secreting arms of the argonaut. 
In the extinct ammonites we haye evidence that the aperture 
was guarded still more effectively by a horny or shelly operculum, 
secreted, in all ae ae 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 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 lnes of growth. Forty-five kinds are enume- 
rated by Bronn; they occur in all the strata in which ammonites 
arefound, and a singlespecimen has been figured by M. D’Archiac, 
from the Deyonian rocks of the Hifel, where it was associated 
with goniatites.t 
Calcareous mandibles, or rhyncholites (FE. Biguet), have been 
obtained from all the strata in which nauwtili 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 Bayaria one 
Fig. 49.4 
* This form was discovered by the late Miss Mary Anning, the indefatigable collector 
of the lias fossils of Lyme Regis, and described by Mr. Strickland, Geol. Journal, vol.i., 
p. 232. Also by M. Voltz, Mem. de I’Institut, 1837, p. 48. 
+ Trigonellites lameliosus, Park. Oxford clay, Solenhofen (and Chippenham), 
associated with ammonites lingulatus, Quenstedt. (= A, Brightii, Pratt). From a 
specimen in the cabinet of Charles Stokes, Esq. 
{ The trigonellites have been described by Meyer as bivalve shells, under the generic 
name of aptychus ; by Deslongchamps under the name of Munsteria. M. D’Orbiony 
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 Z. sanguinolarius, 
found with am. dépressus, 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. Some of them have been 
described by Rolle (1862) as Cyclidia and Scaphanidia. 
§ M. D’Orbigny has manufactured two genera of calamaries out of these nautilus 
beaks (rhynchoteuthis and paleoteuthis). In the innumerable sections of ammonites 
which have been made, no traces of the mandibles have ever been discovered, 
