960 PROF. OWEN ON THE [NoV. 19, 



The perfect specimen (Plate LX. fig. 1) of the last chamber of the 

 Ammonite was derived from that eminently conservative matrix, the 

 lithographic slate of Solenhofen, Bavaria. The extent of the outer 

 curve of the dwelling-chamber to the outer border of its floor (the 

 last septum, x to y) is 4 inches ; the extent of the outer curve of 

 the chambered part of the shell to the inner border of the cham- 

 ber-floor (last septum, y to z) is 6 inches. In the Nautilus pom- 

 ■pilius transmitted in its perfect shell to Prof. Vrolik the corre- 

 sponding admeasurements are 7 inches 9 lines and 11 inches 6 lines. 

 The correspondence in the proportion of the dwelling-chamber to 

 the camerated part of the shell in the Ammonite and Nautilus is 

 thus, as in numerous other instances, shown to be instructively 

 close. 



In the last chamber of Ammonites lingulatus, moreover, as if to 

 proclaim to the most sceptical its function of lodging its constructor, 

 is preserved the only fossilizable part of such (ib. fig. 1, o). 



Since the publication of the ' Paleontologie ' of Pictet 1 , who refers 

 the Trigonellites to the Cirripeds, abundant evidence has been ob- 

 tained of the accuracy of the opinion of Volz 2 , that the Trigonellites of 

 Parkinson 3 {Aptychus of v. Meyer 4 ) were parts of the animal of the 

 Ammonite, and stood in opercular relation to the shell of that extinct 

 Tetrabranchiate. Pictet takes no note of the confirmation contri- 

 buted by Morris to the Volzian examples of Trigonellites within the 

 last or dwelling-chamber of Ammonites, in the portion of an Ammonites 

 walcotti in which " the Aptychus, of a corneo-calcareous nature, 

 was found imbedded in the matrix filling the last chamber about 

 6 inches from the aperture "\ 



This common, though not constant, position led some palaeonto- 

 logists to surmise that the Trigonellites might be parts of the 

 Ammonites' gizzard, like the triturating plates in Bulla lignaria., 

 M. Valenciennes believed them to be lateral supports of the funnel 6 . 

 Van der Hoeven " hazards the opinion that the two juxtaposed fossil 

 shells, known by palaeographs as Aptychus, were two shelly sup- 

 ports of the hood of Ammonites" (Trans. Zool. Soc. vol. iv. p. 22). 

 In this opinion I concur ; but it has been rejected by some experi- 

 enced students of the extinct polythalamous shells. Keferstein and 

 Waagen 7 , e. g., deem the Trigonellites to be sexual characters, and 

 to have served as protective plates of the nidamental glands of the 

 female Ammonite. Waagen adduces, in support of this view, the 



1 Traite de Paleontologie, &c, vol. ii. p. 551, 8vo (1854). 



2 "Sur le Belopeltis" in Memoires de la Soc. d'Hist. Nat. de Strasbourg, 

 tome iii. (1836). 



3 'Organic Eemains of a Former World,' 4to, 1811, p. 1S4, pi. xiii. figs. 9-12. 

 * Nova Acta Acad. Nat. Cur. t. xv. pt. 2, p. 125, pi. lviii. (1831). 



s " Note on Aptychus" in Ann. & Mag. Nat. Hist. vol. x. (2nd ser.), p. 350, 

 pi. v. D. fig. 1. 



6 " Je crois qu'il fant admettre que l'entonnoir de l'ammonite, s'il etait forme 

 de deux valves, ne contenait pas de cartilage interne ; mais que cette piece 6tait 

 remplacee par an organe exterieur compose de deux pieces paires symetriques 

 eomme le sont les aptychus." — Ib. p. 304. 



7 " Ueber die AnsaUstelle der Haftmuskeln beim Nautilus und Ammoniten," 

 Palseontograpliia, 4lo, p. 185, taf. xxxix., 1871. 



