564 



MOLLUSCA 



SUB-KINGDOM VI 



Family 5. Popanoceratidae. Shells similar to Arcestidae (except in primitive 

 Popanoceras, which is discoidal and compressed). Saddles have entire monophyllifc 

 bases, and dorsal sutures are similar to those of Prolecanitidae in sunie <^-iK-ra ; but in 



FIG. 1162. 



Popanoceras multistriatum, Gemm. 

 Permo - Carboniferous ; Sosio, Sicily. 

 2/3 (after Gemmellaro). 



FlG< 1163 ' 



Cydolobus Stachei, Genim. Perrno-Carboniferous ; Sosio 

 Sicily (after Gemmellaro). 



others they become more complex and show approximation to Arcestidae. Antisiphonal 

 lobe trifid, but otherwise entire in primitive forms, becoming complex in specialised 



FIG. 1164. 



Arcestes intuslabiatus, Mojs. Upper Trias ; Stein bergkogel. near Hallstadt, Austria. A, B, External aspect. 

 C, Median section. D, Suture-line. 



genera. Aperture with hyponomic sinus in primitive genera, but acquires faint crests 

 in shells that resemble Arcestidae. 



Stacheoceras, Waagenoceras, Hyattoceras, Gemm.; Popanoceras, Hyatt (Fig. 1162); 

 Parapopanoceras, Haug ; Cydolobus, Waagen (Fig. 1163). Permian and Trias. 



Family 6. Arcestidae. Mojsisovics (pars). Smooth, globose, deeply involute 



