4 6 



LAND AND FRESH-WATER SHELLS. 



'iate ; those shells in which the pallial line is not evenly 

 described, but forms a sinus in relation with the posterior adductor 

 impression are said to be sinupalliate. 



Fig. 14. The anatomy of a Mussel, and the internal markings of its shell. 



7. The lines of shifting of the muscles, numerous lines running 

 from the different impressions towards the umbo. 



8. The hinge, a longitudinal ridge running along the dorsal 



Fig. 15. Right valve of the shell of Cytherea showing internal markings. 



portion of each valve. The absence of teeth in the Anodon 

 shell ; in Unio, Sphceriutn, and Pisidium teeth obtain. The teeth 

 consist of three sets : cardinal teeth situated below the umbones, 



