446 



MOLLUSCA. 



principally by the character just mentioned, but also by the 

 possession of a mesial septum. The Suessia of the Lias, 

 and the Syringothyris of the Carboniferous, are also types 

 very closely allied to Spirifera. 



In the genus Cyrtia the shell resembles Spirifera in most 

 respects ; but the valves are very unequal, the dorsal valve 

 is approximately flat, and the ventral valve is pyramidal, 

 with a very large triangular hinge -area and a long and 

 narrow foramen, which is partially closed by a pseudo- 

 deltidium. Cyrtina (fig. 290, c and D) resembles Cyrtia in 

 the shape of the valves, but the shell is punctate, whereas 

 in the latter it is impunctate. Both genera are Palseozoic, 

 and range from the Upper Silurian to the Carboniferous. 



More important than either of the preceding is the genus 

 Athyris (including under this head the Spirigera of D'Or- 

 bigny), which ranges from the Silurian to the Permian. 

 The shell in this genus (fig. 290, E) is convex, with unequal 



Fig. 290. A, Eetzia serpentina, with part of the dorsal valve removed to show the spires ; 

 B, Uncites gryphus, with the spires (s), from the Devonian ; c, Side view of Cyrtina hetero- 

 clita Devonian ; D, The same viewed from the dorsal aspect; E, Athyris concentricaVevo- 

 nian ; F, Merista Icevis Upper Silurian ; G, Meristella angustifrons Upper Silurian, enlarged ; 

 H, Cast of the same. (After Davidson and Hall.) 



valves, the beak of the ventral valve being incurved, and 

 either perforated by a small round foramen, or having the fora- 

 men concealed or closed in the adult state. (The name Athy- 

 ris, like that of Atrypa, is a zoological misnomer, since in 



