PHYLUM VI 



QQQ MOLLUSCA 



Oprel; (?) Simoceras Zittel (Fig. 1293); Siemiradzkia Hyatt. Type S. (Ämmonites) 



Fio. 1294. 



Peltoceras athleta (Phill.). Upper Callovian ; Vaches noires, 

 Normandy. i/i. 



riiysodoceras circum- 

 spinosum (Oppel). Upper 

 Jura ; Swabian Alps. 1/2. 



bakeriae (d'Orb.) [Terr. Jurass. PL 149, Fig. 1, non PL 148]. Physodoceras Hyatt 

 (Fig. 1295). Type P. (Ämmonites) circumspinosum (Oppel). Upper Jura. 



Family 27. Desmoceratidae ZitteL 



Discoidal moderately involute forms, with simple or divided rihs. These continue 

 without Interruption across the rounded keelless venter. Gonstrictions or varices at 

 regulär intervals. Septa finely digitate, with auxiliary lohes arranged in a straight row. 



The Desmoceratidae are slightly modified descendants of the Stephanoceratidae, 

 and preserve in youth the characters of the ancestral family. 



Subfamily A. Desmoceratinae Zittel (pars). 



Mostly involute shells, smooth, or with constrictions and fold-like costae without 



tubercles that commonly follow the 

 lines of growth across the rounded 

 venter uninterruptedly. Spines 

 sometimes present as in preceding 

 family. Sutures have blunt 

 siphonal saddles, never pointed. 

 First lateral lobes shallower than 

 Fio. 1296. "^ ^^^ Lytoceratidae, the first 



lateral saddles broader, and less 

 deeply cut by marginals. Anti- 

 siphonal lobe long, straight, and 



trifid. Lateral zygous inflections more numerous, and there are often three or more 



zygous dorsal saddles. 



Suture-line of Desmoceras latidorsatum Mich. Gault : 

 Parte du Rhone. 



