187S.3 SHELLS OF CEPHALOPODS. f)(Jl 



correspondence of the relative position of the Aptychus 1 , in the body- 

 chamber, to that of the nidamental glands of Nautilus pompUius 2 to 

 the shell ; and he deems each valve ofApti/chus to have been applied 

 to a lateral lobe of the nidamental gland' 3 . But these lateral lobes 

 are divided from each other by a part of the middle lobe 4 , while the 

 valves of the Aptychus are usually in contact (as in Plate LX. fio-. l ) 

 or may be suturally united along the mid line (as in ib. fic'Vo) 

 But to return to Waagen's argument from position in the fossil shell' 

 In the course of decomposition after death the calcareous plates would 

 be hkely to gravitate or sink deeper into the body-chamber than 

 their natural position in the living Ammonite. So sinking they 

 would rather lodge or settle in the hollow of the outer (ventral) 

 wall of the body-chamber (b) than upon the involute convexity on 

 the opposite (dorsal) wall (a). Moreover there are examples (as in 

 Plate LX. fig. 2) in which the Trigonellites (o, o) have been found 

 in an opercular relation at the mouth of the shell. And, considering 

 the movements to which an Ammonite must have been subject from 

 the time of its death to the solution of the soft parts and final im- 

 bedding of the shell in the matrix or seat of its petrifaction, one is 

 prepared for the rarity of the conservation with the shell of its loose 

 operculum, and for the still more rare retention of the Aptychus in 

 its original position. Waagen admits that there are five specimens 

 of Ammonite in the Munich collection exhibiting this position 5 He 

 however, contends that the breadth of the aperture of the body- 

 chamber is less, in certain Ammonites (A. sterasp>is, e. g.), than the 

 united breadth of the aptychal plates. But so, likewise, if the side 

 lobes of the hood of Nautilus 5 were outstretched horizontally it 

 would exceed the breadth of the outlet of the dwelling-chamber in 

 N. pompihus. But the side lobes of the hood are bent back 

 obliquely in order to close the dorsal side-curved borders, or notches 

 of that part of the shell-aperture, just as the aptychal lobes or 

 valves are bent down in Plate LX. fig. 2, o, o. 



It may be further remarked, in respect to the nidamental 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, 1871. Moreover, in not one of the existing genera or 

 species of Cephalopod, Nautilus included, in which these glands are 

 superadded to the more essential organs of generation, are they en- 

 cumbered in any way or degree with such calcareous plates as Kefer- 

 stein s hypothesis applies to them in the Ammonite. 



1 Pakeoutographia, 1871, taf. xl. fig. 4. 



a See Memoir on the Nautilus, 1832, pi. i., e. 



» Ib. pi. viii. fig. 10, b, b. * lb. ib. a. 



" Unser Museum besitzt gegen 100 exeniplare von Ammoniten mit erlial- 

 tenen Aptychus; unter dieser ganzen Anzahl sind 5, welclie den Aptychus in 

 senkrechterStelluugamEndederWohnkammerliegendhaben" (op cit x> 1991 



6 Memoir on the Nautilus, 1832, pi. iii. fig. 1, g, g. ' l ' J ' 



