THE CEPHALOPODA. 185 



Dr. Waagen,^ in view of clearing away tlie difficulties that surrounded the question, 

 dissected a female NaidilHS pompilius, and has given beautiful drawings of the 

 nidamental gland in that Mollusc, showing the abdominal surface of the Nautilus, 

 the position of these glands, and their relation to the other viscera. After a careful 

 and exhaustive examniation of all the anatomical facts disclosed by his dissection, he 

 concluded that the Aptijchm appears to have belonged to the nidamentary gland of the 

 female Ammonite. 



In his recent memoir on the chambered shells of Cephalopods, Professor Owen^ 

 reviewed in detail the various opinions expressed on the functions of the Aptychm, and 

 remarked " in respect to the uidamental glands, that they are subject to seasonal changes 

 and gain the relative bulk with which the size of the aptychal plates accord only at the 

 period of discharge of the impregnated ova, for which they have to furnish the protective 

 coat or nidus. Such seasonal change is exemplified in the figure of these glands given 

 in the 'Memoir on the Nautilus,' of 1832, and in that which is shown in taf. xix of 

 Waagen's treatise in 1871. Moreover, in not one of the existing genera or species of 

 Cephalopoda, Nautilus included, in which these glands are superadded to the more 

 essential organs of generation, are they encumbered in any way or degree with such 

 calcareous plates, as Keferstein's hypothesis apphes to them in the Ammonite. 



" 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 position, the hood 

 (fig. 24), and which, when the animal had withdrawn into its dwelling, would serve as a 

 rigid defence at the outlet of the shell. ^ 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. The relative size of the Aptyclius agrees with that of the shell. It has been 

 found to measure seven inches six lines in length, and six inches in breadth, in gigantic! 

 Ammonites." [I myself have a specimen collected from the Middle Zone of the Lower 

 Oolite which measures five inches in length and four in breadth, and probably belonged 

 to a large Harpoceras Sowerhyi, which is the only Ammonite with a whorl of that size 

 and shape found in the bed from whence it was collected.] 



" 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 {Aptyclius, v. M.) be preferably accepted the 

 supposition that these parts are calcifications of an Ammonite's hood may be deemed 



1 " Ueber die Ansatzstelle der Haftmuskeln beim Nautilus und dem Ammoniden," ' Palseontographica,' 

 Band xvii, p. 185, pi. xxxix, Cassel, 18fi7 — 1870. 



2 "On the Eelative Positions to iheir Constructors of the Chambered Shells of Cephalopods," 'Pro. 

 of the Zool. Soc. of London,' p. 955, 1878. 



3 'Memoir on the Pearly Nautilus,' p. 12, pi. iii, fig. 1, 1832. 



