CYTHERID^. 



113 





SHAPE AS SEEN LATERALLY. 



TEXTURE AND MARKINGS OF 

 SHELL. 



HINGEMENT. 



CvTHERIDiE — 



Cytherideis 



Elongated, subovate, depressed in 

 front, right valve overlapping 

 in centre of ventral margin 



Smooth, more or less finely punc- 

 tate 



Nearly simple. 



Xestoleberis 



Subtriangular, higher behind than 

 in front 



Smooth and polished, with distant 

 round papillae 



A dentated projecting crest of the left 

 articulating with an excavation of the 

 riS'ht valvp 



Loxoconcha 



Subrhomboidal or " peach-stone 

 shaped " 



Smooth and sparingly papillose, 

 or concentrically pitted 



Two small terminal teeth on each valve, 

 intervening portion often finely 

 crenulated. 



Bythocythere ... 



Subrhomboidal or fusiform with 

 hinder margin produced into a 

 distinct beak 



Smooth or ornamented with small 

 rounded pittings and papillae, 

 often obscurely reticulated and 

 having a ti'ansverse median 

 fissure 



Simnle or cnmnnspd of n Rlip*bt bar and 

 furrow ; no teeth. 



Cytheropteron... 



Subrhomboidal, more or less 

 beaked behind, rounded in 

 front, usually produced laterally 

 towards ventral margin into a 

 wing-like projection ; valves 

 unequal and dissimilar 



More or less pitted ; the pits 

 usually subquadrangular and 

 arranged in transverse rows ; 

 usually with a transverse me- 

 dian furrow 



Two small terminal teeth on right valve 

 with opposing fossse and an inter- 

 vening crenulated ridge on left valve. 



Pseudocy there... 



Obliquelv quadraneular, rounded 

 in front, obliquely truncate 

 behind, and produced into a 

 blunt beak at supero-posteal 

 angle 



Very thin and fragile, smooth, 

 almost structureless 



Simple. 



Cytherura 



Oblong or subtriangular, usually 

 minute, produced behind into 

 a more or less prominent beak, 

 valves dissimilar and unequal 



Smooth, or variously marked with 

 reticulated, ribbed, or punctate 

 ornament, usually robust 



Hinge-processes obsolescent or alto- 

 gether wanting. 



Sarsiella 



Almost circular, rostrate behind 

 (?), much compressed 



Exceedingly rugose, with circular 

 fossse in the interspaces 









Sclerochilus ... 



Elongated, flexuous, bean-shaped 



Smooth and polished, very hard 



Projecting median crest of left valve. 



Xiphichilus 



Valves subequal, compressed, 

 elongated, pointed at both 

 ends, ventral margins much 

 compressed 



Thin and fragile, smooth, polished, 

 marked at margins with trans- 

 verse hair-like lines 



Simple. 



15 



