THE FRESH-WATER MUSSEL 121 



lines of growth seen on the outer surface. If these lines mark annual 

 increments, how old was the animal when it died? What is the oldest 

 part of the valve? This part is called the umbo. What region of the 

 valve shows the most erosion? Why? Examine the broken edge of 

 a valve and find the outer periostracum, the middle prismatic layer, 

 and the inner nacreous layer, or mother-of-pearl layer. Do such sec- 

 tions show layers of growth? Sections ground thin for microscopic 

 study will show further details of structure. 



(d) The shell can be oriented according to the body axes of the 

 animal that occupied it. The hinge is dorsal, the gape is ventral; the 

 valves, therefore, are either right or left. A line through the umbo at 

 right angles to the long axis of each valve divides it into a smaller 

 anterior and a larger posterior portion. Orient the shell in the same 

 position as your own body. 



(e) On the inner surface of the shell near the hinge are the teeth 

 which lock tightly when the valves are closed. What is their function? 

 Roughened places or scars indicate the point of attachment of muscles 

 which close the shell and move the foot. The largest scars are those 

 of the adductor muscles. Which is the scar of the anterior adductor 

 muscle and which of the posterior adductor muscle? Posterior to the 

 scar of the anterior adductor muscle is the scar of the anterior retrac- 

 tor muscle of the foot. Below this is the scar of the protractor muscle. 

 The scar of the posterior retractor muscle lies dorsal to the posterior 

 adductor scar. How does the shell open, since no muscles for this pur- 

 pose are present? Parallel to the margin of the shell and extending 

 between the adductor scars is a line marking the line of attachment of 

 the retractor muscles of the mantle. This is the mantle line, or "water 

 line," often seen on pearl buttons. What is the relation at the edge of 

 the shell of the three layers identified previously? Draw (X 1) the 

 outer surface of the right valve and the inner surface of the left valve 

 to show the features described above. 



(f) Determine the effects of acids and of strong alkali on the shell. 

 Of what is the shell composed? Why are mussels with thick shells 

 found only in regions where limestone rock is abundant? What struc- 

 ture produces the shell? If an extensive collection of shells is avail- 

 able it may be used to illustrate the difference between a genus and a 

 species, and between species of a genus. Examine the shells of a 

 single genus, noting their peculiarities and features that all have in 

 common. Examine other genera and compare with the first. The 

 Genera Quadrula, Lampsilis, and Anodonta are valuable for this 

 purpose. 



