INSTITUTE OF SCIENCE, PHILADELPHIA. 523 



ing; for this no function has been assigned. In Castalia, which is supposed 

 to have two complete siphons, the branchial opening may be complete or in- 

 complete in different individuals of the same species (Ihering). In northern 

 Uniones the outer, in Austral species the inner, gills officiate as a marsupium ; 

 in rare cases both. The specialization of the marsupial gill-sac is often pecu- 

 liar and remarkable. The typical Unio {^Margaritana Schumacher) cannot be 

 generically separated from the forms with " lateral " teeth [Limnium Oken := 

 Uiiio Auct). The sculpture of the tips of the umbones is probably due to the 

 influence of the hooked or serrated edges of the glochidium valves. The 

 group is characteristic of, though not wholly confined to, the northern hemi- 

 sphere. 



Family MUTELID^. 



Shell and anatomy resembling UnionidiZ, without pseudocardinals and 

 laterals; when dentiferous having an irregularly taxodont hinge armature, gen- 

 erally partly-closed mantle lobes, a complete branchial septum, more complete 

 siphons, and with the nepionic stage represented by a Lasidmm, resulting in 

 unsculptured beaks for the adult shell. 



Cretaceous to recent fauna. 



Ex. Leila, Glabaris, Spixoconcha, Iheriiigella, Fossula, Mycetopus, Miitela, Iridiiia, Spatha, 

 etc. 



These types are generally South African, Austral and South American ; 

 Mycetopus is a burrower and shows the usual degeneration of the hinge arma- 

 ture, common to many burrowers, together with a peculiar specialization of 

 the foot for burrowing purposes. Mr. C. T. Simpson states that no Mutelid, 

 known to be such, has the Unionid type of hinge. 



Family ETHERIID^. 



Shell sessile, irregularly modified by adherence to other bodies, nacreous, 

 with a tendency to cellularity of structure ; edentulous; ligament amphidetic, 

 parivincular, deeply sunken, with a large internal resilium, modified by the dis- 

 tortion of the valves ; young regular, equivalve, dimyarian ; the adult irregular, 

 inequivalve, monomyarian (i) or with a very degenerate anterior adductor (2) 

 or with subequal adductors (3); mantle lobes united only for the anal siphon ; 

 foot degenerate or absent in the adult ; branchial septum complete ; gills as in 

 Unionidm ; young byssiferous, larval and nepionic stages unknown; station 

 fluviatile. 



Recent. 

 Ex. I, Mulleria ; 2, Bartlettia ; 3, Etheria. 



The young shell of Bartlettia has well marked nympha; and internal 

 resilium. According to P. Fischer, the nepionic stage is carditiform, but I 

 have never been able to see an uneroded specimen. The relationship of the 

 Naiades to Pteria renders the remarkable resemblance of the adult Mulleria 



