SEA MUSSEL MYTILUS EDUUS. 



135 



Microscopic examination ot the flat surface of one of these flakes near its edge will show 

 distinctly the leaflike layers with their irregular edges (fig. 109). The transverse plane 

 of cleavage always follows the zigzag edges. A surface view of a single lamella under 

 very high magnification reveals a fine network with meshes of a polygonal form (fig. 1 10). 



The ligament when examined in cross section with an ordinary hand lens presents 

 three distinct layers; the outer portion is marked by the dark brown periostracum, the 

 middle part is composed of a yellow-brown homogeneous substance, while the inner 

 layer is of the same color but marked with numerous horizontal dark brown lines. 

 Under higher magnification four distinct and separate layers are visible. 



The outermost layer consists of the periostracum (fig. 113, Per, p. 136), which is 

 really not a part of the ligament proper, although it is fused so closely with it as to form 



108 



110 



Fig. 108.— Transverse section through prismatic layer of shell, showing needlilike prisms, which are hild together by an 

 organic matrix of conchiolin. X 854. 



Fig. 109. — fragment of nacreous layer showing overlapping lamclhc. X soo. 

 Fig. 1 10.— Surface view of nacreous layer very highly magnified. 



a unit body. What appears to be the middle layer under low magnification is divisible 

 into two sharply separated parts when examined with stronger lenses. The outer por- 

 tion of this layer presents a homogeneous structure similar to a film of gelatin, with a 

 yellowish color often tinged with blue or green (fig. 113, HyL). The inner part of the 

 middle layer is of a darker shade and strongly granular in appearance, due to an abundance 

 of irregular masses of lime crystals, many of which are aggregated into starlike bodies. 

 A few of these crystals may be seen scattered in the homogeneous layer close to its bor- 

 der next the granular layer (fig. 113, GL). The inner layer is distinguished from the 

 others by its cross-striped appearance (fig. 113, IL). It is marked by numerous fine 

 vertical lines across which there run at right angles many dark brown bands of varying 

 width. Crystals of lime salts are present in small numbers scattered throughout the 

 substance of this layer. The crystals may be readily removed by treating the sections 



