962 PROF. OWEN ON THE [Nov. 19, 



In the application of the anatomy of the constructor of the Pearly 

 Nautilus to the solution of the problem of the nature and function 

 of the Trigonellites, I was led to regard them as the homologue of the 

 organ, or a portion of the organ, in Nautilus, which is "of a fibrous 

 texture, resembling dense corium," called from its shape and posi- 

 tion the "hood" (Plate LX. fig. 3, e), and which, when the animal 

 had withdrawn into its dwelling, "would serve as a rigid defence at 

 the outlet of the shell" l . It needed only that this part should be 

 more or less calcified to form the preserved portions of an operculum 

 like that ascribed to the Ammonite. If, for example, calcification 

 had commenced in each half of the symmetrical " hood," and had 

 stopped at the mid line (where the hood is thinnest), the pair of the 

 there-often-suturally-joined symmetrical pieces of the Aptychus would 

 have resulted. The relative size of Aptychus agrees with that of the 

 shell. It has been found to measure 7 inches 6 lines in length and 

 6 inches in breadth in gigantic Ammonites 2 . It may be doubted 

 whether the nidamental glands ever increased in the same ratio ; and 

 it is still less likely that they needed such defensive plates in their 

 season of rest and attenuation. 



If, therefore, my homology of the symmetrical halves of the 

 Nautilus' hood with the parial Trigonellites {Aptychus, v. M.) be 

 preferably accepted, the supposition that these parts are calcifica- 

 tions of an Ammonite's hood may be deemed reasonable. That the 

 fibrous basis of the hood was retained in different degrees in the 

 Ammonites is indicated by the simply corneous or chitiuous condi- 

 tion of the Aptychus which has been preserved in some examples of 

 Ammonites falcifer, Sow., and its allies. In other species, as in 

 the Ammonites lingulatus (Plate LX. fig. 1, o), the lateral calcifica- 

 tions have partially met and joined at the mid line ; in a third series 

 these opercular plates are thicker and are there suturally united. 

 This is the case in the small or young specimen of Ammonites sub- 

 radiatus, Sow., in the British Museum, which is described and figured 

 by S. P. Woodward, F.G.S., in 'The Geologist' 3 . As the view of 

 the specimen there given is an oblique side one, I here append a 

 direct view of the aperture of the dwelling-chamber as closed and 

 protected by its operculum (Plate LX. fig. 2, o). The correspond- 

 ence of general shape with that of the " hood " of the Nautilus 

 pompilius, as figured in plate iii. fig. 1 of my • Memoir,' is close 4 . 



The conjoined plates of the Aptychus (ib. fig. 2, o, o) form a tri- 

 angular disk, of which the base is backward, excavated to receive the 

 involute part of the shell, j), with the sides of the base, like the 

 corresponding lobes of the " hood," bent down to cover the laterally 

 extended parts of the wider terminal coil of the shell, q. Even in 

 the contrast between the papillose wrinkled outer surface and the 



1 Memoir on the Pearly Nautilus, 1832, p. 12, pi. i. n, and pi. iii. fig. 1. 



2 See de Zigno, Memorie del R. Istituto Veneto, torn. xv. tab. viii., 1870. 



3 8vo, 1860, p. 328. 



4 Woodward gives the following description of its ammonitic homologue : — 

 "The operculum is flat in the middle, with a slight furrow along the suture, 

 and is much bent down at the hinder corners, where it abuts against the inner 

 whorl of the shell. It is sculptured externally with about twelve angular con- 

 centric furrows ; the inner surface is smooth." — Ib. ib. 



