162 MYAD7E. 



of their greatest development, were abundant inhabitants 

 of warm seas, and, within the British area at least, gradu- 

 ally diminished in numbers as more temperate conditions 

 prevailed. Now, however, some of the most characteristic 

 forms are among the most arctic of mollusca. 



MYA, LiNN^us. 



Shell more or less oblong or rhomboidal, equi valve, 

 gaping at the extremities. External surface of the valves 

 more or less furrowed or striated transversely, and fur- 

 nished with a wrinkled epidermis, which is continued over 

 the mantle and tubes of the animal ; beak depressed ; 

 hinge composed of a dilated, ascending spathulate tooth in 

 the left valve, connected by a short thick internal liga- 

 ment with a corresponding socket in the right. Muscular 

 impressions distant, well-marked, and connected by a deep- 

 ly sinuated palleal impression. 



Animal oblong, with long tubes, enclosed in a strong 

 case-like coriaceous epidermis. Mantle closed in front, ex- 

 cept anteriorly, where there is an opening for the passage 

 of a small conical foot. Siphons united nearly to their extre- 

 mities, Avhich have fimbriated orifices. Labial jmlpi striated. 

 The genus Mya^ as originally constituted, included mol- 

 lusks of very different families, whose shells presented 

 similarities in the construction of the hinge. It is now a 

 limited and very natural group, represented by few, but 

 well-marked species, most of which are inhabitants of the 

 temperate and colder seas of the northern hemisphere. 

 They are all borers in mud and sand, where they live 

 buried beneath the surface in an erect position, their 

 hiding-places being indicated by holes corresponding to the 

 extremities of their tubes. They are excellent articles of 



