XL] THE FRESH-WATER MUSSEL. 135 



and an anterior larger and posterior smaller 

 end : note the soft uncalcified ventral edge of 

 each valve. 



The umbo; a small blunt eminence on the 

 dorsal border of each valve near its anterior 

 end. 



c. The ligament: an elastic uncalcified part of the 

 exoskeleton behind the umbones, uniting the 

 two valves and tending to keep their ventral 

 edges slightly separated. 



d. The markings on the shell. 



~^a. External markings. The outside of the shell 

 is greenish brown, and on it are seen a num- 

 ber of concentric lines generally parallel to 

 the margin of the shell, and more numerous 

 towards the ventral edge. 



J (3. Internal markings. The interior of the valve 

 is white and iridescent : on it are seen, near 

 the dorsal border, two oval marks, the ante- 

 rior and posterior adductor impressions. 



Joining the two adductor impressions is 

 a curved line, the pallial impression, which 

 marks where the muscles of the edge of the 

 mantle were fixed to the shell. 



In front of the posterior adductor impres- 

 sion is seen a small mark, indicating where 

 the posterior retractor muscle was fixed. 



Behind the anterior adductor impression 

 are two marks, one opposite its upper, the 

 other opposite its lower end : the former 

 indicates the point of attachment of the 



