LAMELLIBRANCHIATA : PHOLADID/E. 



495 



, OR LANTERN-SHELL FAMILY. - - This Fam- 

 ily embraces acephala which have the shell thin and often 

 inequivalve. There are less than one hundred living spe- 

 cies, and four hundred fossil. Fig. 472. 



GASTROCILENID^E. -This Family comprises acephala 

 which have the shell thin, and gaping ; often cemented 



Fig. 473. Fig. 474. 



Gastrochcena uiodiocena, 

 Lam. Galway. 



~ _ -_-,!. , U 1 ..':. 5 



Asfergillum vaginiferiutt, Lam. One half. Red Sea. 



into a shelly tube when adult. There are twenty or thirty 

 living, and as many fossil species. 



PHOLADID/E, OR PHOLAS AND SHIP- WORM FAMILY. - 

 This Family embraces acephala which have the shell 



Fig. 476. 



Fig. 475- 



Pkolas Bakeri, Desh. One half. 

 India. 



Pholas crispata, Linn. 

 New England and eastward. 



open at both ends, thin, white, exceedingly hard, and 

 armed with rasp-like imbrications. They burrow in al- 

 most all substances. Fifty or sixty species are living, and 

 as many more are fossil. Marine. 



SUB-SECTION II. 



THE ORDER OF TUNICATA, OR ASCIDIANS. 



THIS Order comprises acephalous mollusks which have 

 no hard parts, but which are protected by an elastic tunic 

 instead of a shell. Some are transparent, so that their 

 whole internal structure may be easily seen. Several 

 families, and quite a large number of species, are known. 



