LECTURES ON MOLLUSCA. 269 



family relationships can always be determined by the microscopic ex- 

 amination of any small fragment; the prismatic structure not being 

 seen in the Fan-shells. 



The animal of the Pearl-oysters has the mantle free all round, ex- 

 cept where the flaps are joined, in the middle, by the attachment of 

 the gills. The edges are beautifully fringed. The lips are plain, and 

 rather small. There is only one principal adductor muscle in this and 

 the remaining families of the Pectinibranchs ; although there are often 

 seen other small scars, formed by the foot-muscles and the retractors 

 of the mantle. The foot is finger-like and grooved, working through 

 a notch at the side of the shell, and spinning a byssus, which in Pinna 

 is long and silky, but in other genera is horny and rather solid. 



All the Aviculids which have been observed in the young state have 

 the pointed shape of the Mussels, which is permanent in the Pinnas. 

 These creatures, which are sometimes two feet long, stick their pointed 

 beaks in the sand or mud, with the knife-like edges of their gaping 

 shells projecting upwards. These are sometimes dangerous to navi- 

 gation. They differ from the ordinary Wing-Shells in having the 

 small anterior adductor somewhat developed. A little crab (called 

 "Pinna-guardian" by Aristotle; perhaps the mollusk calls it Pinna- 

 plague) is fond of nestling in its breathing cavity. Fossil species are 

 found from the Devonian age; some of the thick oolitic forms being 

 grouped as Trichites. 



The typical Avicula tribe have thin, slanting shells, swollen in the 

 middle, and produced on each side of the hinge into wings which are 

 some times very long, but greatly vary in the same species. They are 

 fond of mooring themselves to Gorqonias, floating wood, and other 

 light bodies. One valve is generally larger than the other; and there 

 are small hinge-teeth as in Alasmodon. The fossil, species are very 

 numerous, beginning from the earliest rocks. 



The Pearl-oysters, (Margaritiphora,) have heavy shells with short 

 wings, having thick layers of "mother o'pearl," beautiful wherever 

 it is worked. The pearls themselves are formed by excrescenses or 

 deposits of pearly matter in the mantle, often taking form from sand 

 or other extraneous substance which has been introduced. Nearly 

 three hundred tons of this shell are yearly imported into England. 

 They have no hinge teeth. In this respect they resemble the Ham- 

 mer-oysters, (Malleus,} which take the contrary extreme of shape. 

 The body and the side-wings being all very long and narrow, the 

 shell takes the form of a T. In the young shells, which are often 

 regarded as distinct species, the side wings are not developed. The 

 shape then resembles the ^Vulsella, which lives embedded in sponge, 

 and has the ligament concentrated in a spoon-shaped cavity. Some 

 of the early fossil forms have been grouped as Ambonychia, Cardiola, 

 and Eurydesma. Mo?iotis and Halobia are from the Triassic rocks. 

 The Silurian Pterinea and the oolitic Pteropema have few or numer- 

 ous anterior teeth,, and long posterior teeth as in Unio. The ancient 

 Posidonomya has a thin, earless shell, without teeth. 



In the remaining group of this family, the young shell is like Avi- 

 cula, but in the adult the ligament is fixed into numerous pits along 

 the hinge line. The name Perna, given by Lamarck to the common 



