GENUS IV. MY A. 51 



(/)/. 1./. 10. Z>) ; when in its natural position in the sand, 

 this is the upper end, from which the tubes project. When 

 there are any lines or marks on the shell nearly parallel to 

 the margin, which is opposite to the hinge and beaks, they 

 are called transverse ; but if there are any diverging- like 

 rays, from the beak to the margin, lliey are called longi- 

 tudinal. 



Lamarck has divided this genus into four, as follows: 

 Unio. Shell transverse, having three muscular impressions ; 

 an irregular callous hinge ; tooth prolonging itself on 

 one side under the cartilage, and articulating with that 

 of the opposite valve (see Schrot. Flus. Conch, t. 2. /. 3. 

 or Mya Pictonim, pi. 2. f. 12). 

 Glycemeris. Shell transverse, gaping at both ends ; the 

 hinge callous, without teeth ; cartilage external (see 

 Mya Siliqua, Chemn. vol. xi. t. 198. f. 1934). 

 Mya. Shell transverse, gaping at both ends; cartilage in- 

 ternal ; the left valve furnished witli one hinge tooth, 

 compressed, rounded, perpendicular to the valve to which 

 tlic cartilage is attached (see Mya truncata, pi. 1. 

 /. 10). 

 Vulsella. Shell free, longitudinal, nearly equivalved, with 

 a flattish callous hinge, without teeth, projecting alike 

 on each valve, with a conical rounded hollow for the 

 cartilage, terminating in a very short bent beak (see 

 Mya Vulsella, pi. 1. /. 11). Animal fixing itself with 

 a byssus. 



H 2 



