52 HANDBOOK OF SHELLS. 



the siphons are proportionately longer, and in many cases, too, 

 the valves do not meet at this point when the shell is closed. 



Attention to these particulars is necessary when arranging 

 your bivalves, as on them their classification depends, the class 

 being divided into 



a. ASIPHONIDA (Stphonless). 



b. SIPHONIDA Intcgro : $allialia (with Siphons). Pallial-line 

 entire. 



c. SIPHONIDA Sinu-pallialia (with Siphons). Sinus in pallial- 

 line. 



DIVISION a. ASIPHONIDA is next subdivided into 



1. The Ostreidfc, or oysters, which are deservedly a distinct 

 family in themselves. 



2. The AnomiadcE, comprising the multiform and curiously 

 constructed Anemia, with the "Window-shells" (Placiina], 



3. The Pectinidce, taking its name from the genus Pecten, or 

 " Scallop-shells," of which one kind (P. maximus) is frequently 

 to be seen at the fishmongers' shops. The " Thorney oysters " 

 (Spondylus) take rank here, and are highly esteemed by col- 

 lectors, one specimen indeed having been valued at ^25 ! 



4. The Aviculidcc, or "Wing- shells," among which are num- 

 bered the "Pearl-oyster" of commerce (Meleagrina mar- 

 gar it if era). The strange T-shaped " Hammer oyster " belongs 

 to this family, as does also the Pinna. The Pinnas, like the 



mussels and some 

 other bivalves, 

 moor themselves 

 to rocks by means 

 of a number of 



Fig.,,. 



mollusc, and termed the byssits, which in this genus is finer, 

 more silky, than in any other, and has been woven into articles 

 of dress. 



5. The Mytilida, or mussels, including the Lithodomus^ or 

 "Date-shell," which bores into corals and even hard limestone 

 rocks. 



6. The Arcada, or "Noah's-ark-shells," characterized by 

 their long straight hinge-line set with numerous very fine teeth 

 (Fig. 17). The " Nut-shell" (Nitaila) belongs to this family. 



