2o8 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, o.p) : hence the name " Trifora " given to Lamellibranchia 

 which exhibit this arrangement. The second orifice is called the 



JMTlttO 



cf^ lunuh 



Fio. 189. 

 Left valve of Mvretrix, 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, o.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 Kelly a, 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, o"). 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 Asper- 

 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 



