xi.]" 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 



v. ' 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. 



(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 



