612 



MOLLUSCA 



PHYLUM VI 



hyponomic sinus and special inflections known as " brachial sinuses " are formed by bases 

 of the arms on edges of the brachial area. Finally, the aperture becomes reduced to a more 

 or less Y- or T-shaped figure, with an open semicircular sinus at the end of the hyponomic 

 slit or area, and similar sinuses in the edges of the brachial slit, corresponding to the 

 number of arms. Silurian. 



Mandaloceras Hyatt {Dimorion Barr.) (Fig. 1133). Differs from Poterioccras in 

 the gerontic aperture, which is laterally contracted, and has hyponomic and brachial 

 areas distinctly differentiated in all but the most primitive species. More specialised 

 forms have these areas narrowed down, but sjjecial sinuses are not formed. 



Trimeroceras Hyatt {Trimorion, Trimeres Barr.) ; Pentameroceras, Septameroceras 



Hyatt. Silurian. Aperture in the first has a median and 

 two brachial ; in the second a median and four brachial ; 

 and in the last a median and six brachial sinuses. 



Fio. 1133. 



Mandaloceras (Gom'phoccraf:) hohcmicum (Barr.). Silurian (Etage B) ; 

 Dvoretz, Bohemia. A, Side view of conch. B, Aperture. 



Fig. 1134. 



Tetrameroccras panderi 

 (Barr.). Silurian (B) ; Dvoretz, 

 Bohemia, i/i (after Barrande). 



Hemiphragmoceras Hyatt. Compressed endogastric cyrtoceracones having a 

 narrowed hyponomic area like Phragmoceras, but with brachial areas as in Dimeroceras. 

 Type H. {Phragmoceras) piisillum (Barr.). 



Tetrameroccras Hyatt {Tetramorion, Tetrameres Barr.) (Fig. 1134). Like the last^ 

 but with more highly contracted aperture and four lateral sinuses. 



Hexameroceras Hyatt. Brachial area with six lateral sinuses. Octameroceras 

 Hyatt. Brachial area with eight lateral sinuses. Type 0. (Phragm.) callistoma (Barr.). 



Family 10. Phragmoceratidae Hyatt. 



Smooth hreviconic cyrtoceracones and gyroceracones rapidly expanding by growth in 

 their dorso-ventral diameters, and having open apertures only in primitive types or the 

 young and ephebic stages of more specialised forms. In the latter gerontic apertures are 

 laterally contracted and have co very long hyponomic area terminated by a large 

 hyponomic sinus. The brachial area may be more or less open and ellipitical, or 

 narrowed and transversely elongated, but always has an entire outline. Siphuncle 

 generally large, nummuloidal, and often has actiniform lamellae and endosiphuncles in 

 later stages. Shells mostly endogastric. 



Godoceras Hyatt. Excessively short and rapidly expanding cyrtoceracones like 

 some species of Rizoceras, Init with large living chambers, narrow venter, and large 

 .siphuncle just ventrad of centre. Aperture constantly open. Type G. {Gyrtoceras) 

 domitum (Barr.). Silurian. 



Protophragmoceras Hyatt (Fig. 1136). Similar to the last, but form more com- 



