LAMELLIBRANCH1ATA : PHOLADID^. 495 



ANATINID.E, 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. 



GASTROCHJENID.E. This Family comprises acephala 

 which have the shell thin, and gaping ; often cemented 



Fig. 473- Fig. 474. 



Gastrochana modiocena, Aspergillum vaginiferunt, Lam. One half. Red Sea. 



Lam. Galway. 



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- 



Pholas Bakeri, Desh. One half. Pholas crispata, Linn. 



India. 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 

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

 * Now regarded as a class 



