604 



MOLLUSCA 



PHYLUM VI 



Shells with longitudinal ridcjes and sometimes annular folds in the young, hut often 

 smooth in the ephehic stage. Sutures ivith ventral, lateral ^and dorsal lohes, or almost 

 straight Siphuncle nummuloidal and often dorsad of centre. Annular lohes knoimi to 

 he present in specialised forms. 



Rhadinoceras, Nephriticeras Hyatt. Devonian. 



Family 14. Trigonoceratidae Hyatt. 



Gyroceracones and nautilicones having at some stage or throughout life trigonal 

 volutions, a more or less concave venter, and generally fluted shelL Sutures with ventral 

 saddles in the young, hecoming divided hy shallow lohes in later stages, and in some genera 

 the dorsal lohes of the young hecome divided suhsequently hy dorsal saddles. Gerontic 

 living chamher occasionally free near the aperture. Annular lohes ohserved in only one 

 species (Apheleceras disciforme). Young have longitudinal ridges roughened hy transverse 

 hands as in Thoracoceras. Siphuncle small, ventrad of centre. 



Trigonoceras M'Coy ; Goelonautilus Foord {Trematodiscus Meek ; Trematoceras 



Hyatt) ; Suhclymenia d'Orb. ; Strohoceras, 

 Apheleceras, Diorugoceras, Ephippioceras 

 Hyatt. Carboniferous. All nautilicones 

 but the first, which is gyroceraconic. 



Family 15. Triboloceratidae Hyatt. 



Gyroceracones and nautilicones similar 

 to Trigonoceratidae, and with concave 

 venter at an early stage or until late in 

 life. The venter afterwards hecomes more 

 or less elevated, and in most species convex. 

 Sutures also similar, hut annular lohes 

 are present in all the nautilicones save 

 Coloceras. 



Fig. 1119. 



Vestinautütis konincki (d'Orb. ). Lower Carboniferous ; 

 Tournay, Belgium. Oral and lateral aspects of young 

 individual, with umbilical Perforation. 



Triholoceras Hyatt; Vestinautilus Ryckh. (Fig. 1119); Planetoceras, Stearoceras, 

 Goloceras Hyatt. Carboniferous. 



Family 16. Rhineceratidae Hyatt. 



Gyroceracones and nautilicones like Thoracoceras in nepionic stage, hut suhsequently 

 hecoming Uangular in section, and generally developing solid, more or less tetragonal, 

 volutions. Longitudinal ridges and flutes also developed, hut are more uniform in size 

 than in the preceding family, and venter is always convex. Annular lohes present in 

 all nautilicones so far as Jcnown. 



Ehineceras, Lispoceras, Thrincoceras, Phloioceras, Discitoceras {Discites M'Coy), 

 Leuroceras, Phacoceras Hyatt. Carboniferous. 



. VI. Hercoceratida Hyatt. 



^ Primitive shells have projecting hands of groivth and processes similar to those of 

 primitive Ryticeratida, hut less numerous, heing present in only one row, and evolving 

 more rapidly into nodose or symmetrical, spout-like, spinous processes: More specialised 

 forms are tuherculated as in Ryticeratida, hut there are never more than three rows of 

 nodes on either side, and these are regiUarly distrihuted — one on the umhilical Shoulder, 

 another on the ventro -lateral angle, and the third dose to the median ventral line. 



