208 THE LAMELLIBRANCHIA 
in many Unionidae the anal orifice is itself divided into two, the more 
anterior aperture being dorsal and the more posterior—considered 
topographically—being the anal opening proper. In addition to the 
first fusion, there is a second in the Ledidae among the Protobranchia, 
in the Dreissensiidae and the Mutelidae among the Submytilacea, and 
in all other Eulamellibranchia and the Septibranchia (Fig. 249). In 
Castalia, a member of the Mutelidae, it has been observed that the 
second fusion may be present or absent in different individuals. 
This second fusion of the pallial borders is always pretty close to the 
first (Fig. 241), and forms the boundary of an orifice almost in juxta- 
position to the anal aperture, leaving in front a third orifice 
(Fig. 241, 0): hence the name “ Trifora” given to Lamellibranchia 
which exhibit this arrangement. The second orifice is called the 
Yl host 
“ \iumpres 2 
Fic. 189. 
Left valve of Meretriz, from the inner face. (From Lankester, after Owen.) 
branchial or inhalant, and the third the pedal orifice. The last-named 
is generally very large (Figs. 194, 221, 0.p, etc.), but its extent is 
always in inverse ratio to the extent of the second fusion and in 
direct ratio to the size of the foot protruded through it (Figs. 219, 
246). In the genus Kellya,- however, it is not the third or anterior 
aperture that serves for the passage of the foot, but the second or 
ventral aperture (Fig. 187, 0”). Finally, when the second fusion 
extends for a long distance, that is to say, when the foot is very 
anterior, rudimentary, or cylindrical, a fourth pallial aperture may 
be formed between the pedal and branchial apertures, and therefore 
within the region of the second fusion. This disposition may be seen 
in some Solenidae, in Lutraria and Glycimeris, and in sundry Anati- 
nacea, such as Myochama, Chamostraea, Thracia, Pholadomya, and A sper- 
gillum, which are sometimes called, on this account, ‘‘ Quadrifora.” 
This fourth orifice probably arises as a subdivision of the pedal 
orifice ; it is placed far forward in Solen, and is both inhalant and 
