626 



MOLLUSCA 



PHYLÜM VI 



some species from certain localities, inside the living Chamber and close to the 

 ^ j^ venter (Figs. 1159, 1161), 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 furnished 

 with aptychi.^ 



Aptychi are composed of three layers, of which the 



middle one is the thickest and exhibits a cellular struc- 



ture, whereas the two outer layers are comparatively 



(Fig. 1164). Detached aptychi have been 



Fig. 1160. 



Anaptychi. A, Amaltheus Clense 

 nnatus Brug. 

 B, Goniatites uchtensis Keys 



.spiruttus Bvng.ua.8.yy classificd by Zittel into several groups according to 



their structure. Cellulosi (Fig. 1163) are smooth, thick 

 plates, with punctate external surface ; Imhricati (Fig. 1159) have ^ the 

 surface traversed 

 by oblique folds 

 or costae; Pundati 

 (Fig. 1164, C) 

 have rows of 

 punctae and over- 

 lapping folds; 

 Granulosi include 

 thin plates having 

 the external sur- 

 face covered with 

 concentric folds or 

 rows of tubercles 

 or spinules ; Bugosi 

 are thick plates 

 with irregularly 

 arranged granules 

 or rows of nodes on the outer surface 



Fig. 1161. 



Harpoceras lythense (Sowb.). Upper Lias ; 

 Boll, Würtemberg. Aptychus in the living 

 Chamber. 



Fig. 1162. 



Scaphites spiniger Schlüter. 

 Upper Cretaceous ; Coesfeld, 

 Westphalia. Detached 



aptychus. 



and Cretaceous. 



Nigrescentes (Fig. 1161) are covered 

 with a thin carbonaceous coat- 

 ing; and Coalescentes (Fig. 1162) 

 have the two thin plates fused 

 along a median depression. This 

 last is a phylogerontic condition 

 of the aptychus occurring in 

 Scaphites. 



Classification. — Leopold von 

 Buch prepared the way for a 

 general Classification of the 

 Ammonoidea by pointing out 

 three grand divisions which he 

 called "genera." These were 

 the Paleozoic Goniatites, Ceratites, 

 from the Trias and Cretaceous, 

 and Ammonites, from the Jura 

 Von Buch's chief distinctions were based on the outlines of 



Fi(i. 1163. 



Aptychus laevis v. Meyer. Litliographic Stone ; Solenhofeii 

 Bavaria. A, External aspect; B, Internal, i/i. 



1 Michael, R., Zeitschr. deutsch, geol. Ges., 1894, vol. x\\\.—Retowski, 0.. 

 1891, vol. ii. — Blackmore, IL P., Geol. Mag., 1896, dec. 4, vol. iii. 



Neues Jahrb. 



Min.J 



