50 GENUS IV. MYA. 



their tubes when extended to their full length, always main- 

 taining a counnunication with the water. 



Linna?us has described 7 species, Gmelin has added 14, and 

 Dr. Turton 5. Total, 26. 



Mya Perna is so much like the common Muscle, that it 

 cannot be properly placed in this genus. Linnaeus says, in 

 the Systema Naturce, twelfth edition, " Forte Mytili species ;" 

 and it is said to have been his intention * to have formed a 

 new genus for it by the name of Perna, and also another by 

 the name of Unio, for the reception of Mya Pictorum and 

 margaritifera, which have an external cartilage, and longi- 

 tudinal ridges in the place of teeth (p/. 2. /. 12) ; the animals 

 of these, as well as tiie teeth and cartilages, being so very 

 different from the generic description, and from the other 

 species of Mya, that the propriety of removing them cannot 

 justly be doubted. Mr. Montagu has formed a new genus, 

 part of which he has taken fi-om the Mya, and part from the 

 Mactra, by the name of Ligula, wliich he describes thus : 

 " Shell bivalve, equivalve ; hinge with a broad tooth in each 

 valve, projecting inwards, furnished with a pit or cavity for 

 the reception of the connecting cartilage ; and in some species 

 a minute erect tooth. The shells he places here are Mya 

 prcEtenuis, pubescens, distorta ; Mactra compressa, tenuis, and 

 Boysii, &c. 



The gaping or truncated end is called the posterior part 



* See Linn. Trans, vol. 7. p. 205. 



