632 



MOLLUSCA 



I'HYLUM VI 



Family 7. Prolecanitidae Hyatt. 



The youny Uve « long undivided ventral lobe. Pnm^t^ve forms are compreesed, 

 discoidJ,aJmore epecialised gmera become invoUte and as^ume a ^od^fied ^^rcesjean 

 a^pect. Saddles aremtire in the former, but thefirst latcraU become very large and are 

 Jbdivided by simple marginal lohes. Lateral lobes entrre m pr^m^t^ve genera and 

 become bijid or trifid in specialised forms, but rarely have more nnmerous d^g^taUons. 

 Antüiphonal lobe entire or pointed. Siplmnele withont ealcareous sheath ; funnels pro- 

 siphonate so far as hnown. 



yVVW^\ 



•^vAllnjUR/UK 



Fig. 1180. 



Belocerus muüüobatum (Beyrich). Upper 

 Devoiiian ; Adorf, Westphalia. 



Fig. 1181. 

 Proleeanites lunulicosta (Sandb.). 

 Upper Devonian ; Nassau (after 

 Sandberger). 



Subfaiiiily A. Prolecanitinäe Frech. 



Shells discoidal or evolute, coiupressed or siibquadrate in section. Primitive forms 

 with undivided ventral lobes, and rounded saddles and lobes of the lecanitean type. 

 More specialised shells have entire hastate lobes and saddles, and similar but divided 

 ventral lobes. Aperture with well-marked hyponomic sinus. Shells smooth or 

 Costated, and often with longitudinal ridges. 



Phenacoceras Frech ; Proleeanites Mojs. (Fig. 1181). Devonian to Garl)oniferous. 



Subfamily B. Noritinae Karpinsky. 



Similar to Prolecanitinäe, but the ventral lobe instead of becoming divided in the 

 usual way, retains the larval trifid stage throughout life in primitive species. In 

 specialised forms the larval siphonal saddles enlarge in the neanic stage, thus building 

 up a Single siphonal saddle with a comparatively large siphonal lobe. First lateral 

 lobes may be bifid, trifid or completely serrated in specialised shells, and the second 

 and other lateral lobes also may become ceratitic. The saddles, however, retain more 

 or less of their primitive outlines, and their bases are entire. Sutures with adventitious 



