CYTHERIDtE. 



113 



Cytherid^ — 

 Cytherideis 



Xestoleberis 



Loxoconcha 



Bythocythere 



Cytheropteron . 



Pseudocythere . 



Cytherura 



Sarsiella 



Sclerochilus 

 Xiphichilus . 



SHAtE AS SEEN LATERALLY. 



Elongated, subovate, depressed in 

 front, right valve overlapping 

 in centre of ventral margin 



Subtriangular, higher behind than 

 in front 



Subrhomboidal or " peach-stone 

 shaped " 



Subrhomboidal or fusiform, with 

 hinder margin produced into a 

 distinct beak 



Subrhomboidal, more or less 

 beaked behind, rounded in 

 front, usually produced laterally 

 towards ventral margin into a 

 wing-like projection ; valves 

 unequal and dissimilar 



Obliquely quadrangular, rounded 

 in front, obliquely truncate 

 behind, and produced into a 

 blunt beak at supero-posteal 

 angle 



Oblong or subtriangular, usually 

 minute, produced behind into 

 a more or less prominent beak, 

 valves dissimilar and unequal 



Almost circular, rostrate behind 

 (?), much compressed 



Elongated, flexuous, bean-shaped 



Valves subequal, compressed, 

 elongated, pointed at both 

 ends, ventral margins much 

 compressed 



TEXTURE AND MARKINGS OF 

 SHELL. 



Smooth, more or less finely punc- 

 tate 



Smooth and polished, with distant 

 round papillae 



Smooth and sparingly papillose, 

 or concentrically pitted 



Smooth or ornamented with small 

 rounded pittings and papillae, 

 often obscurely reticulated and 

 having a transverse median 

 fissure 



More or less pitted ; the pits 

 usually subquadrangular and 

 arranged in transverse rows ; 

 usually with a transverse me- 

 dian furrow 



Very thin and fragile, smooth, 

 almost structureless 



Smooth, or variously marked with 

 reticulated, ribbed, or punctate 

 ornament, usually robust 



Exceedingly rugose, with circular 

 fossse in the interspaces 



Smooth and polished, very hard 



Thin and fragile, smooth, polished, 

 marked at margins with trans- 

 verse hair-like hnes 



HINGEMENT. 



Nearly simple. 



A dentated projecting crest of the left 

 articulating with an excavation of the 

 right valve. 



Two small terminal teeth on each valve, 

 intervening portion often finely 

 crenulated. 



Simple, or composed of a slight bar and 

 furrow ; no teeth. 



Two small terminal teeth on right valve 

 with opposing fossae and an inter- 

 vening crenulated ridge on left valve. 



Simple. 



Hinge-processes obsolescent or alto- 

 gether wanting. 



Projecting median crest of left valve. 

 Simple. 



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