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, Hntovalva, 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 
Fic. 188. 
Arca lactea, Linnaeus, ventral aspect. a.a, anterior adductor ; a.l, anterior lip ; a.p, anterior 
labial palp ; }, byssus ; /, foot ; g’, internal gill-plate ; g’’, external gill-plate ; g.a, gill-axis ; h.a, 
posterior adductor; m, mouth; pa, mantle; p./, posterior lip; re, rectum; v.c, visceral com- 
missure ; v.g, visceral ganglion. (After Deshayes.) 
other words, ina 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 
