TERN A. 515 



The species of this genus are most numerous in the 

 temperate and Arctic seas, several being found on the 

 shores of Britain, and others in Nova Zembla and 

 Ochotsk, while a few have been observed in New Zealand. 

 They range from low-water to forty fathoms. Some of the 

 species are found imbedded in the mantle of Ascidians, as 

 ft marmorata, and others have been observed to spin a 

 sort of nest, or are found concealing themselves among the 

 roots of sea-weed and corallines. Fossil species have been 

 detected in the Upper Green-sand of Europe. 



Species of C rev ell a. 



bulla, Dkr. glaudula, Totten. 



decussata, Mont. rhombea, Berkl. 



divaricata, D'Orb. 



Sub-gen. modiolaria, Beck (Lanistes, Swai7is., not Montf. 



Lanistiua, Gray). 



Shell obloug, the radiating striae obsolete in the middle. 



Cbenuana, Becluz. laevigata, Gray. 



coarctata, Dkr. marmorata, Forbes. 



concinna, Dkr. nana, Dkr. 



cor, Martyn. nexa, Gould. 



costulata, Risso. nigra, Gray. 



cultellus, Desk. pectinula, Gould. 



Cumingiana, Dkr. pusilla, Gould. 



discors, Linn. strigata, Hani. 



faba, 0. Fabr. verrucosa, Midd. 



Genus PERNA, Adanson. 



Margins of mantle simple, open ; anal tube short, more 

 or less perfect. 



