with Descriptions of the ?iexv Species. 



371 



II. Ma'ctra Linn. 

 Shell ovate trigonal, subangular at each end. Cardinal teeth two in each 

 valve ; the front tooth of the left valve, and both of the right valve, thin, 

 compressed ; hinder of the left valve trigonal, folded. The lateral teeth 



distinct, anterior and posterior. Siphonal inflection ovate, distinct. Liga- 

 ment external, in a more or less oblique triangular groove, separated from 

 the cartilage pit by a distinct shelly septa. 



A. The lateral Teeth subequal, laminar, thin, moderately distant from the Um- 

 bones and the Hinge Teeth. Hinge marginal, double-edged. 



* Lunule and Lozenge smooth. 



1. Mdctra glauca ; M. helvacea Chemn. 



2. Mdctra stidtorum Linn., var. white. 



3. Mdctra maculata Lam. 



** Lunules concentrically grooved. Lozenge smooth. 



4. Mdctra discors Gray. Shell ovate, trigonal, rather solid, ventricose, 

 white. Umbones nearly close, with two diverging reddish lines. Lozenge 

 rather flattened, rugulose, margined by slightly raised lines, with two 

 or three raised concentric lines. Ligament very small. 



Inhabits . 



Much the habit of a Mulinia, but grooved in front. 



*** Lunule and Lozenge concentrically grooved. 



§ Shell thin. Umbones rather separate. Ligament diverging. 



5. Mdctra tumida Chemn.; M. turgida Lam. The anterior tooth T-shaped; 

 the anterior part arising from the middle of the front side of the hinder 

 part. 



6. Mdctra grdndis Lam. (flg. 30.) 



7. Mdctra orndta Gray. Reddish, pale-rayed, and white-dotted. Um- 

 bones bright red. 



Inhabits China. 



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