ORDER II AMMONOIDEA 545 
similar to the hood of Nautilus, and protected the animal when it was retracted 
into the living chamber. When composed of a single 
piece, the plate is called an anaptychus ; in such cases it 
is invariably carbonaceous, and was doubtless horny in 
the living animal (Fig. 1111). The anaptychus is rare 
in the Palaeozoic, and has not yet been found in the 
Trias, but occurs among the Arietidue and Amaltheidae 
of the Lower Jura. The operculum, when formed of 
two plates, is termed an aptychus, and is always of ie. TAL 
calcareous composition. It is noteworthy that these  anaptyeni. 4, Amattheus 
plates occur uniformly in the same position among ‘SPinatus, Brug. Lias. Th 
some species from certain localities, inside the living 
chamber and close to the venter (Figs. 1110, 1112), a circumstance that led 
Waagen to suppose they 
served to protect the nida- 
mental gland of female 
shells. As shown by 
Michael in Oppelia, even 
the embryonic 
shells were fur- 
nished with 
aptychi.! 
Aptychi are 
composed of 
three layers, of 
= JOU which the middle 
Fic. 1113. oneisthethickest 







Fig. 1112. bes Liki See : 
Scaphites spiniger, Schliit. . > j 
Harpoceras Lythense, Sowb. sp. Upper Lias ; Upper Cretaceous ; ; Coesfeld, and exhibits a 
Boll, Wiirtemberg. Aptychus in living Westphalia. Detached cellular struc- 
chamber, aptychus. 
ture, whereas the 
two outer layers are comparatively dense (Fig. 1115). Detached aptychi 
have been classified by Zittel 
into several groups according to 
their structure. Cellulosi (Fig. 
1114) are smooth, thick plates, 
with punctate external surface ; 
Imbricati (Fig. 1110) have the 
surface traversed: by oblique 
folds or costae; Punctati (Fig. 
1115, C) have rows of punctae 
and overlapping folds ; Granulosi 
include thin plates having the 
external surface covered with 
concentric folds or rows of 
tubercles or spinules ; Rugosi dick 
: c c Aptychus luevis, v. Meyer. Lithographic Slates ; Solenhofen, 
.are thick plates with irregularly Bavaria. 4, External aspect ; B, Internal, 1/;. 
arranged granules or rows of 
nodes on the outer surface ; Nigrescentes (Fig. 1112) are covered with a thin 

1 Michael, R., Zeitschr. deutsch. geol. Ges., XLVI. p. 697, 1894.—Retowski, O., Neues Jahrb., 
I]. p. 220, 1891.—Blackmore, H. P., Geol. Mag. [4], III. p. 529, 1896. 
VOL: I 2N 
