BIVALVE SHELLS. 463 



rough and hairy, or ciliated, or velvety, or scaly, or smooth, 

 dull, or "listening. The periostracum of the genus Trigona 

 is partially covered with a velvet-like silvery coat as if it had 

 been washed over with whitening. Dr. Fleming discovered 

 that this coating was formed of siliceous spicula similar to 

 those of the siliceous sponges. The true nature of the coat- 

 ing is scarcely determined, but Mr. J. E. Gray thinks that 

 it is formed hj the animal as it produces the periostracum, 

 and that, consequently, it is a constituent part of the shell.* 



The use of the periostracum appears to be to defend the 

 shell from the attacks of boring worms. Hence, it has been 

 remarked that it is ordinarily hairy only in such species as 

 do not bury in the sand or mud, and which, therefore, are 

 more exposed to their enemies than the arenicolous genera. -j- 



The external surface is variously adorned, and furnished 

 with colours, stria?, grooves, ribs, tubercles, scales, and spi- 

 nous and foliaceous processes. When these are arranged in 

 the direction of the shell's length, they are said to be longitu- 

 dinal ; when in the opposite direction, they are transverse, or 

 concentric ; and when they cross the two directions they are 

 oblique. In general, the valves are exactly similar in colour- 

 ing and sculpture ; but there are many exceptions. Of 

 fixed shells, the lower valve differs very often in colour from 

 the upper, the first being white or colourless, while the other 

 may be gaily coloured. The Spondylus and Pecten afford 

 many remarkable instances of this. In sculpture, "a difference 

 is rare, but it is exemplified in a common native Tcllina, in 

 which the striae of the two valves are differently disposed. 

 Ohs. In some Univalves, the sculpture of the young shell 

 differs from that of the old, or mature one. " In the young 

 of Plekocheilus the whole shell is without striaj, and is beau- 

 tifully corroded on its surface, the plicae being smooth and 

 distinctly marked, as they are added by the fresh operations 

 of the pallium. In the young of the other snail (Bulimulus 

 undulatus) we observe numerous fine longitudinal and trans- 

 verse striae, which it will be vain to look for in the parts 

 added by the animal as it advances in strength ; a circum- 

 stance that would have led to the multiplication of species, 

 had not specimens been discovered in various stages. "J 



The most remarkable parts observable on the external sur- 

 face, are the beaks, the lumde, or areola, and the corselet. 

 (Fig. 85.) 



* Ann. and IVIag. N. Hist. ser. 2, iv. 296. 



t Encyclop. Method. Vers. i. 96. Adanson's Senegal, 120. 



1 Rev, L. Guilding- in Zool .Tonvn. iii. 533. 



