524 



MOLLUSCA 



l- vIXGDOM \I 



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

 forms are tuberculated as in Ryticeratida, but there are never more than three rows of 

 nodes on either side, and these are regularly distributed one on the umbilical shoulder, 

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

 Annular lobes absent except in a few Triassic forms. Siphuncle generally more or less 

 nummuloidal. 



Family 17. Hercoceraticiae. 



Cyrtoceracones, gyroceracones, nautilicones, and torticones having depressed elliptical, 

 sub-quadrate, or trapezoidal sections. Aperture has two deep sinuses with projecting edges 

 at the ventro-lateral angles, and these are usually persistent, forming two lines of more or 



FIG. 1071. 



Ptenocerus (Gyr.) vl 

 Barr. sp. Silurian 



Hercoceras minim, Barr. Silurian (Etage G) ; Hlubocep, Bohemia (after Konieprus, Bohemia. 

 Barrande). (after Barrande). 



J/i 



t-UJce processes. Sutures with ventral, lateral, and dorsal lobes. Siphuncle 

 ventrad of centre. 



Hercoceras, Barr. (Fig. 1071); Trochoceras, Barr.; Ptyssoceras, Ptenoceras (Fig. 

 1072), Anomaloceras, Hyatt. Silurian. 



Family 18. Tainoceratidae. 



Discoidal nautilicones with more or less massive volutions which at some stage or 

 throughout life are trapezoidal in section, tuberculated, and without well-defined lateral 

 and umbilical zones. Sutures have ventral, lateral, and dorsal, but no annular lobes. 

 Spinous processes are complete, never spout-like. Siphuncle small, tubular. 



Temnocheilus, M'Coy (Endolobus, Meek ; Cryptoceras, d'Orb.). Section trapezoidal 

 throughout lii'e, and one row of persistent spines and nodes on either side at the 

 ventro-lateral angles. Devonian to Carboniferous. 



Foordiceras, Hyatt. Permian. Metacoceras, Coelogasteroceras, Hyatt. Carbon- 

 iferous. Diadiploceras, Hyatt. Devonian. Tainoceras, Hyatt. Carboniferous. 



Family 19. Centroceratidae. 



Gyroceracones and nautilicones with young similar to early stages of Temnocheilus 

 before the impressed zone is formed. Shell subsequently becoming tetragonal in section, 



