38 THE SHELL. 



so-called hood (iv, 62), by which the animal may close the 

 aperture of the shell, and in the body-chamber of many of 

 the Ammonites (possibly secreted by these dorsal arms) there 

 are opercular-shaped bodies (ii,38. The true nature of these 

 shelly or flexible hornj^ plates has not been authoritatively settled 

 however ; they have been described under the names of Apt3'chus 

 and Miinsteria as bivalve shells, and have, also, been thought to 

 be cirripeds and even products of the neck-cartilages, of the 

 nidimental gland, gizzards, or ventrally placed cuttle-bones! of 

 Ammonites. In the Arietes group of Aminonites the aptychus 

 is a single, horny, flexible piece, whilst in other groups it is 

 shelly, consisting of two plates joined by a median suture, the 

 exterior face smooth or striated and the interior marked by 

 growth-lines. Prof. Waagen has recentl}^ adopted the theory 

 first advanced by Keferstein,and afterwards supported by Zittel, 

 that the aptychi were connected with the nidimental gland, and 

 he has even proposed a classification of Ammonitidse based upon 

 the absence or presence, and peculiarities when present of these 

 bodies. 



If, as Ferussac first suspected, the Ammonite was a dibran- 

 chiate cephalopod, with the shell like that of the Spirula more 

 or less completely covered by the posterior portion of its body, 

 instead of being a tetrabranchiate with external shell like the 

 Nautilus, then the difficulty of accounting for the presence of 

 the aptychus is increased, if that body is truly a product of 

 the nidimental gland ; moreover, the outer laj^er of the shell 

 estimated as epidermal in character, must be otherwise explained. 

 It is difficult to imagine a shell internal, that is provided with 

 the spinous processes of some of the species, and hard to 

 reconcile the weight and size of sundry " cart-wheel " Ammo- 

 nites with the idea of an internal shell. Fischer, however, 

 appears disposed to take this view of the subject, following- 

 Hyatt and other recent investigators. For myself, I prefer 

 leaving Ammonites among the tetrabranchiates, and, conse- 

 quently, considering its shell as external, and that the aptychus 

 is essentially an operculum, or a product of the " hood " which 

 exists in Nautilus, and may have similarly existed in Ammonites. 



Yon Jhering finds in the aptj^chus a possible product of the 

 cartilaginous neck-button of dibranchiate decapod Cephalopoda 

 — that is, supposes them to be a product of a similar organ; and 

 upon this theory he would consider them dibranchiates instead 

 of tetrabranchiates. The use of the neck-button is not so 

 apparent in an animal having a heavy external shell for its 

 protection, and if the shell be considered internal, as in Spirula, 

 and the animal also similar, it would be diflicult to imagine how 

 the aptychi occur in the body-chamber. 



The outer la^^er of the shell has been generally destroyed in 



