MOLLUSCA. LAMELLIBRANCHIA 



207 



Fig. 95. Section of pris- 

 matic layer of recent 

 Pinna, parallel to the 

 surface of the shell and 

 at right angles to the 

 prisms. Magnified. 



thin membranous sheath, which can be isolated by dis- 

 solving the calcareous part of the 

 shell in acid. The external sur- 

 face is covered by a green or 

 brownish horny layer (c), the peri- 

 ostracum (frequently referred to as 

 the ' epidermis '). The prismatic 

 layer is formed by the margin of 

 the mantle only; the pearly 

 layer by the general surface of 

 the mantle, and this layer grad- 

 ually encroaches on the former, 

 which consequently cannot after- 

 wards increase in thickness, 

 whereas the pearly layer may do 

 so throughout the life of the animal. The pearly layer is 

 absent in many forms, and the prismatic structure of the 

 outer layer may be indistinct or altogether wanting, and 

 this layer has then a porcellanous appearance. Sometimes 

 the shell consists entirely of aragonite or of calcite ; in 

 other cases one layer may be of calcite and the other of 

 aragonite. 



The surface of the shell may be smooth, or may be 

 ornamented with radiating or concentric ribs and striae, or 

 with tubercles, or spines. Often the exterior shows con- 

 centric lamellae, which represent periods of growth. The 

 part of the shell at the umbo is that which was first 

 formed, and often differs in ornamentation and form 

 from the other parts. The margins of the valves may be 

 smooth or crenulated ; sometimes, as in some species of 

 Pecten, the entire shell is corrugated, thus increasing its 

 strength without materially adding to the weight. In 

 many genera the two valves can be completely closed, in 



