80 



MANUAL OF THE MOLLUSCA. 



The margin nf the aperture is quite simple in the recent nautilus, and 

 affords no clue to the many curious modifications observable in the fossil 

 forms. In the ammonites we frequently find a dorsal process, or lateral pro- 

 jections, developed periodically, or only in the adult (fig. 55, and pi. III., fig. 5), 



In phragmoceras and gomphoceras (figs. 40, 41) the aperture is so 

 much contracted that it is obvious the animal could not have withdrawn its 

 head into the shell like the nautilus. 



Fig. 40. Gomphoceras. Fig. 41. Phragmoceras.* 



M. Barrande, from whose great work on the Silurian Formations of 

 Bohemia these figures are taken, suggests that the lower part of the aperture 

 (s s) which is almost isolated, may have 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. 43, k), 

 formed by the union of the two dorsal arms, which correspond to the shell- 

 secreting sails of the argonaut. 



In the extinct ammonites we have evidence that 

 the aperture was guarded still more effectively by a 

 horny, or shelly operculum, secreted, in all probabi- 

 lity, by these dorsal arms. In one group (arietes,) 

 the operculum consists of a single piece, and is horny 

 and flexible.f In the round-backed ammonites the 

 operculum is shelly, and divided into two plates by a 

 straight median suture (fig. 42). They were de- 

 scribed in 1811, by Parkinson, who called them tri- 

 gonellites, and pointed out the resemblance of their Fig. 42.1 



* Fig. 40. Gomphoceras Bohemicum (Barrande), reduced view of the aperture ; *, 

 the siphonal opening. Fig. 41. phragmoceras callistoma (Barr.) both from the U. 

 Silurian, Bohemia. 



t This form was discovered by the late Miss Mary Anning, the indefatigable col- 

 rector of the lias fossils of Lyme Regis, and described by Mr. Strickland, Geol. Journal, 

 vol. I., p. 232. Also by M. Voltz, Mem. de 1'Institute, 1837, p. 48. 



% Trigonellites lamellosus, Park. Oxford clay, Solenhofen (and Chippenham ; ) as- 

 sociated with ammonites lingulatus, Quenstedt. (= A. Brightii, Pratt). From a speci- 

 men in the cabinet of Charles Stokes, Esq. 



