516 



MOLLUSCA 



SUB-KINGDOM VI 



Aphragmites, Barr. Only gerontic living chambers known ; these are similar to 

 those of Ascoceras, but have no internal sigmoidal dorsal saddles. Silurian. 



Ascoceras, Barr. (Figs. 1057, 1058). Gerontic living chambers 

 internally contracted by the formation of large sigmoidal saddles, 

 and septa more or less incomplete ventrally. Siplmncle with 

 funnels only in the young, the collars in ephebic stages becoming 

 immnmloidal and often incomplete in old age. Aperture open. 

 Silurian. 



Glossoceras, Barr. Known only by gerontic living chambers 

 which are like those of Ascoceras, except that aperture has dorsal 

 and lateral crests. Silurian. 



Volborthella, Schmidt. Minute orthoceracones with conical 

 septa, small siphuncle, perfectly plain upon the surface of the 

 septa. Living chamber flaring and imcontracted. Lower Cam- 

 brian ; Finland, Esthonia. St. John's Group ; Nova Scotia. 





FIG. 1057. 



Ascoceras manubrium, 

 Lindstrom. Silurian ; 

 Gottland. 1/2- (Re- 

 stored after Lindstrom.) 



FIG. 1058. 



Ascoc-eras bohemwum, Barr. Silurian (6tage E) ; Kozorz, Bohemia. .4, 

 Specimen with shell partially preserved. B, Cast of living chamber detached 

 from preceding portion. C, Longitudinal section, w, Living chamber ; 

 c 1-4, Camerae ; I 1-4, Saddles contracting the living chamber. 1/1 (after 

 Barrande). 



Family 2. Mesoceratidae. 



Depressed elliptical cyrtoceracones, known only by their gerontic living chambers, and 

 affinities therefore uncertain. They are globular at this stage, and have highly contracted, 

 transversely elongated, and approximately dumb-bell-shaped apertures. 



Mesoceras, Barr. Aperture with very shallow hyponomic sinus. No internal 

 gerontic sigmoidal septa. Silurian. 



Billingsites, Hyatt. Aperture without hyponomic sinus. Gerontic living cha ml > r 

 partly filled by dorsal sigmoidal saddles as in Ascoceras, but septa complete on the 

 ventral side. Silurian. 



Sub-Order C. SCHISTOCHOANITES. Hyatt. 



Funnels usually more or less imperfect, present on the internal side, and absent or 

 split on the outer side. 



This group will be better understood after the publication of Professor W. B. 



