MOLLUSCA— CEPHALOPODA. 37 



24f . Venter rounded or flattened, with median keel well de- 

 veloped s. 



s. Keel continuous, not bounded by groove 15^. 



15''. Ribs curved sigmoidally, often noded. 



CLXX. Schloenbachia . 

 \^\ Ribs bending forward and ending before reaching 

 keel — nodes faint or absent. 



C LX X 1 1 1 . Prion ocyclus . 



s. Keel continuous, bounded by grooves iS''. 



16'', Whorls scarcely embracing, ribs continuous, gen- 



iculated at ventrolateral margin o'. 



o''. Young subquadrate, tuberculated. 



CXXXIV. Coromceras. 

 o^. Young ovoid, higher than wide. 



CXXXV. Arnioceras. 

 \6\ Whorls moderately embracing, ribs broken up 



into nodes (continuous in old age) s. 



s. Keel discontinuous in adult — ribs ending in spines at 



ventrolateral margin CLXXII. Prionotropis. 



24f. Venter acute with pronounced keel t. 



t. Keel strongly corded, shell with numerous fine bifurcat- 

 ing and curving ribs CXXXIX. Cardioceras. 



t. Keel sharp, broken by costae into tubercles — ribs coarse, 



with nodes CLXIX. Barroisiceras. 



24f . Venter channeled, without median keel, channel bounded 



by tubercles C LI I . Metoicoceras. 



vv. Surface smooth or with discontinuous ribs or nodes 25f. 



25+. Shell strongly compressed and closely involute u. 



u. Venter acute, sharp ; suture comparatively simple,. 17''. 

 \']^ . Young with grooved venter. 



CLXV. Metengonoceras. 



17''. Young not grooved CLXVI. Sphenodisrus. 



u. Venter channeled ! 18'. 



18''. Suture very complex, channel with or without 



marginal nodes CLXVII. Placenticeras. 



18''. Suture relatively simple (almost ceratitic) p^. 



p''. Channel bounded by small ridges. 



CLXIII. Protengonoceras. 

 p''. Channel flat or concave, generally bounded by 

 elongate, alternating nodes. 



CLXIV. Engonoceras. 



25f. Shell not compressed, venter broadly rounded, surface 



smooth or with coarse folds or tubercles; joung Placenti- 



ceroid. CLXVIII. Stantonoceras. 



3333. Close-coiled except last portion in which the whorl is partly 

 loose-coiled, with generally a final rectangular curve. 



CXXXI. Scaphiles. 



3333. Loose-coiled throughout; generally costate ww. 



WW. Form a regular loose spiral, the whorls not in contact ; with 

 two (or three) rows of nodes CLIIL Crioceras. 



