MACTRA 



TESTA plerumque tenuis, aequivalvis, subaequi- 

 latera, subtrigona, lateribus paululum hians: 

 dente eardinali in utraque valva compresso, 

 plicato-canalieulato, cum foveola adjecta intus 

 prominula; dentihus later alihus^ alterius valvae, 

 duobus anticis, duobus posticis, alterius, uno 

 antieo, uno postieo. Impressiones musculares 

 duse, laterales. Impressio pallii adbaerentis 

 musculi sinu mediocri. Ligamentum duplex, 

 majus internum, minus externum. 



The greater part of the Liiinean Mactrae are here sepa- 

 rated from the LiUrariee and Anatin^^, retaining, however, 

 the Linnean generic appellation. We have not hesitated 

 to alter the character of the Genus given by Lamarck, 

 because we did not think it sufficiently descriptive of the 

 peculiar lateral teeth of the shells composing it. Some 

 have thought that the whole Genus might be divided into 

 two; first, the thicker species whose lateral teeth are 

 striated; and second, the thinner shells whose lateral 

 teeth are plain : the former of these seem to us to be inter- 

 mediate between the true MactrcB and the NucuI(B; but 

 we have not ventured to separate them from the Macfrce, 

 inasmuch as in the present state of our knowledge of 

 their animals, we are not aware of any difference. 



Shell mostly thin^ sometimes rather thick, seldom 

 very thick; equivalve, generally nearly equilateral; for 

 the most part more or less regularly triangular; slightly 

 gaping at one end, at the other scarcely, if at all. 

 One cardinal tooth in each valve, which is folded into 



