254 E. H. L. Schu'arz — Shell- Structure of Ammonites. 



Fig. 3. Section of Am. [Grammoceras) r«(?i«ws, Schloth., from Bridport, stLOwing" 

 tlie fourth wliorl with a granular layer, a, interposed between it and 

 the next ; b, deposit in the keel ; c, siphuncle with the outer sheath, 

 d, N.B. the shell-wall is not continued over the back of the smaller 

 whorl. 



Fig. 3«. A section of the fifth whorl showing the deposit [b) in the keel splitting 

 to form the hollow keel. 



Fig. 4. Section of the junction of the seventh and eighth whorls in Am. [Ainalthetm) 

 margaritatus, Schloth., showing the wrinkled layer. At a it has been 

 pushed away from the inner shell ; b, deposit in the keel, which does 

 not split in this species. 



Fig. 5. Portion of the same enlarged, a, a, nacreous layer of the larger whorl 

 terminating abruptly at b ;, c, nacreous layer resting on the apices of 

 the wrinkled layer, and which is not continuous with that of the sides 

 of the shell ; d, d, d, sections of the septa. 



Fig. 6. Three wrinkles enlarged, from the same. 



In forms with a hollow keel, constituting the group Falcoideii of 

 V. Schwarz,^ the space included in it is commonly considered to be 

 bounded on the inner side by the prismatic layer, and on the outer 

 by the porcellanous one.- I have not been able to examine the 

 typical case of Am. [Oxynoticeras) oxynotum, Quenst., as all our 

 English specimens are represented as casts ; but what I have said 

 about the doubtful nature of the so-called outer layer seems to 

 throw suspicion on its ever so occurring. In Am. [Grammoceras) 

 radians, Schloth , however, in about the third whorl, a thin layer of 

 prismatic tissue arises in the keel, quite separate fi'om the usual 

 nacre ; this grows in thickness as the shell increases in size, till 

 about the fifth whorl it splits, and encloses a triangular cavity which 

 eventually becomes the hollow keel in the adult. The cavity, then, 

 is bounded by its own peculiar paries (&), and is really a separate 

 tube closed at the end, and has all the nacreous layer external to it 

 (see Fig. 3a). In Am. [Amaltheus) margaritatus, Schloth., a similar 

 deposit occurs, but does not split. Whether the hollow keel is 

 invariably formed in this waj^ I have not examined sufficient 

 material to state. 



An analogous phenomenon is the case of hollow tubercles whose 

 cavity is cut off from that of the air-chambers ; in casts, therefore, 

 the prominences are not represented. Such is the case in Am. 

 (Liparoceras) Henleyi, Sow., where the prismatic layer divides, and 

 a thin bridge of nacre separates the two cavities. 



That the chambers do contain gas, I think, is proved by those 

 of the Nautilus containing a gas differing from ordinary air in its 

 greater percentage of nitrogen ^ ; and their use, I think, may be 

 understood from the habits of their living relative, the octopus,* 

 which are in all j)robability similar. This latter when it darts 

 upon anything, such as a crab falling through the water, shoots 

 backwai'ds just above it, and the prey is caught in the swirl 

 following in the animal's wake, and is easily surrounded by its 

 arms. It would obviously be an advantage if the hinder portion, 



1 Inaug. Dissert. Tiibingen, Salzburg, 1873. 



2 Hyatt, Arietidse, p. 216; Hang, Neues Jahrb., 1885, pt. iii, p. 593. 

 ^ Vrolik, Ann. Mag. Nat. Hist., xii, 1843, p. 173. 



* Steinmann, Berich. d. Natm-forsch. Gesell., Freiburg, 1889, vol. iv. 



