THE LAMELLIBRANCHIA 



207 



concrescence of the internal reduplicature of their margins ; this 

 union may be localised in one, two, or even three more or less 

 extensive regions. Thus, there is only one line of union in the 

 Solenomyidae (in which it is long, Fig. 231), the Aviculidae, the 

 Ostraeidae, Entovalva, Scioberetia, the Mytilidae, the Carditidae, the 

 Astartidae, the Crassatellidae, the majority of the Lucinidae, 

 the Unionidae (Fig. 242), and in certain species of Cyrenidae ; in 



f 



Area lactea, Linnaeus, ventral aspect. a.a, anterior adductor ; a.l, anterior lip ; a.p, anterior 

 labial palp ; b, byssus ; /, foot ; g', internal gill-plate ; g", external gill-plate ; g.a, gill-axis ; h.a, 

 posterior adductor ; m, mouth ; pa, mantle ; p.l, posterior lip ; re, rectum ; v.c, visceral com- 

 missure ; v.g, visceral ganglion. (After Deshayes.) 



other words, in a very large number of Submytilacea. This single 

 line of union is always posterior, and is the prime factor in the 

 formation of an aperture opposite to the anus (Fig. 231, a.or] known 

 as the exhalant pallial or "anal" orifice. It serves for the evacua- 

 tion of the excrements, of the water which has been used for respira- 

 tion, etc., and is, in the manner indicated, entirely cut off from the 

 rest of the pallial aperture through which the respiratory and food- 

 bearing currents of water find an entrance, and through which the 

 foot is thrust out. Lamellibranchs with this disposition of the pallial 

 border have received the name of " Bifora." It should be noted that 



