INVERTEBRATE PALEONTOLOGY. 145 



strongly, incurved; lunule obsolete; hinge with three cardinal and one 

 remote posterior lateral teeth in each valve ; muscular impressions distinct, 

 those til' the pedal muscles being connected with the upper cuds of the 

 anterior adductor scars ; pallia! line well defined, simple, or sometimes sub- 

 truncated under the posterior muscular impression. 



In this genus, the posterior cardinal tooth of the right valve is strong, 

 very oblique, and provided with a more or less distinct mesial longitudinal 

 furrow; while just in front of it there is a rather large, sometimes narrow, 

 subtrigonal pit for the reception of the middle cardinal of the other valve. 

 The anterior upper half of this pit is bounded by the comparatively small, 

 compressed, middle tooth, directed more or less obliquely forward and down- 

 ward. A little in advance of, and obliquely below, this middle tooth, and 

 separated from il by a longitudinal furrow, or notch, is the small, usually 

 subtrigonal, anterior cardinal of this valve; and in front of the latter occurs 

 the more or less elongated pit for the anterior cardinal of the other valve. 



In the left valve, the posterior cardinal tooth is narrow and very obliquely 

 elongated; while the middle cardinal is short, more or less compressed, sub- 

 trigonal, and ranges nearly vertically. In" front of the latter, the anterior 

 cardinal is seen presenting an oval or- slightly-elongated form, and ranging 

 nearly at right angles to the middle tooth, parallel to the cardinal margin, so 

 as to present somewhat the appearance of the anterior lateral in some types 

 of Qallista. The posterior lateral teeth are elongated parallel to the margins 

 of the valves, that of {lie lefl valve fitting in a furrow above that id" the right. 

 These lateral teeth, as well as the anterior cardinal tooth of \ he left valve, 

 are sometimes marked with fine, regular cross-stria', or the latter more or 

 less corrugated. 



We sometimes see Palaeozoic shells referred to this genus, but it is 

 almost morally certain that none of them really belong to it. A. few of the 

 Upper Triassic and some Jurassic species may possibly fall into this genus, 

 but it is more probable that a critical comparison of their hinge-characters 

 would show that most, if not all of them, present generic differences. In the 

 Cretaceous, however, we do meet with specigs agreeing exactly, in all generic 

 characters, so far as it is possible to determine by the shell alone, with this 

 genus. Most of the Cretaceous species, however, that have been figured by 

 d'Orbigny and others under the name Cyprina, show differences in the hinge- 

 characters that place them properly in other allied groups. 

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