98 



SUPPLEMENT TO THE 



Genus — Athyris, M'Coy, 1844 = Sjjmgera, D'Orbigny, 1847. 

 Sup., PI. V, figs. 14 to 19. 



The internal characters of this important genus are now well known. It will be 

 desirable to take the characters of the genus from Mr. Glass's admirably prepared 

 specimens of the Carboniferous Athyris plano-sulcata of Phillips. The first attach- 

 ment of the spirals takes place at the hinge-plate {a) ; thence the two principal stems 

 proceed for a short distance downwards to <5, then they suddenly bend backwards, 

 forming a broad rounded curve facing the bottom of the dorsal valve {b, d, e). The 

 primary lamellae between the hinge-plate and the point b are slightly curved. The con- 



Athyris plano-sulcata. Carboniferous ; Castleton, Derbyshire. Developed 

 by the Rev. Normau Glass. 



vexity being towards the ventral side. Erora b, the primary lamellge increase in width, 

 this increased width continuing to a little less than half their length at d. At this point 

 each principal stem gives forth a lamella {d, h,) which projects into the middle space inter- 

 vening between the two spirals, and here the lamellae are connected together and become 

 expanded, and roof-shaped {k). This roof-shaped projection is broader in some specimens 

 than in others. Prom the upper portion of the expanded roof- shaped projection (/i) a 

 curved lamella (/) extends upwards, and bifurcating at its extremity gives off on either 

 side a lamella {y). The accessory lamella (q) curves upwards and backwards to the 

 inside of the primary lamella, the course of which it follows to d, and, though it is entirely 

 free from attachment, it is much closer throughout to the first than to the second 

 coil. The accessory lamella is very narrow at its commencement, but it afterwards 

 increases in width and is slightly narrowed again before its termination. 



The main features of the above description are the same as those which I have pre- 



