666 



MOLLUSCOIDEA. 



impunctate. The genus Cyrtia is Silurian and Devonian, a familiar 

 species being the C. exporrecta of the Wenlock Limestone, while the 

 species of Cyrti7ia range from the Devonian to the Trias. 



More important than either of the preceding is the genus Athyris 

 (including under this head the Spirigera of D'Orbigny), which 



Fig. 527. — A, Retzia 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 heteroclita 

 — Devonian; d, The same viewed from the dorsal aspect; e, Athyris concentrica — Devonian ; 

 f, Merista Icevis — Silurian; g, Meristella angustifro7is—'$>\\\ix\2Ln, enlarged; H, Cast of the 

 same. (After Davidson and Hall.) 



ranges from the Silurian to the Trias. The shell in this genus 

 (figs. 527, e, and 528) is convex, with unequal valves, the beak of 



the ventral valve being incurved, 

 and either perforated by a small 

 round foramen, or having the 

 foramen concealed or closed in 

 the adult state. (The name 

 Athyris, like that of Atrypa, is 

 a zoological misnomer, since in 

 both genera the beak of the ven- 

 tral valve is really perforated, in 

 the young state at any rate. Some 

 authorities, however, retain the name of Spirigera for those forms 

 in which the foramen remains throughout life, and employ that of 

 Athyris for those in which this aperture becomes closed in the adult 

 condition, the latter having the additional distinctive character that 

 the interior of the dorsal valve is partially divided by a longitudinal 

 septum.) The spiral supports for the arms in Athyris are largely 

 developed, and their pointed extremities are directed towards the 

 lateral angles of the shell (fig. 528, c). 



Fig. 528. — Athyris subtilita. — Lower Car- 

 boniferous. The right-hand figure shows the 

 interior of the dorsal valve, with the spiral 

 supports for the arms. (After Dawson.) 



