938 PELECYPODA. [Eulamellibranchia. 



Distribution. South America, Australasia, South Africa. 

 Fossil from the Trias. 

 Vernacular Name. Pearl-mussel. 

 Maori. Kakahi. 



Subgen. 1. HYRIDELLA, Swainson, 1840. 



Hyridella, Swainson, Treatise on Malac., 1840, 285. Type : Unio australis, 

 Lamarck. 



Animal having the embryos occupying the inner gills for the most 

 part, which are united for their entire length to the abdominal sac ; 

 outer gills pointed below in the middle ; palpi triangular ; branchial 

 opening papillose ; anal opening smooth, not separated from the 

 superanal opening. 



Beaks rather low, sculpture consisting of curved, generally nodu- 

 lous ridges, which approach below but usually have a smooth area of 

 shell between them ; surface sulcate or sometimes corrugated and 

 nodulous ; epidermis rayless ; teeth rather delicate, compressed, often 

 somewhat rudimentary. 



Distribution. New Guinea, Australia, Tasmania. New Zealand. 



KEY TO SPECIES. 



A. Shell thin, umbones at about the anterior fourth of length. 



a. Shell without trace of radial sculpture, with threadlike con- 



centric striae, not distinctly winged posteriorly . . . . Lessoni. 



b. Shell with faint indications of radial sculpture, dorsal and 



ventral margins subparallel . . . . . . . . lutulentus. 



c. Shell typically winged, but some varieties have the posterior 



dorsal margin horizontal ; with fine radiate striae and 



nodules, which are sometimes prominent, V-shaped . . Menziesi. 



B. Shell thick and solid, heavy, the umbones at the anterior fifth to 



seventh of length ; radiate sculpture faint . . . . Zelebori. 



1. Diplodon Lessoni, Kiister, 1856. Plate 59, figs. 1, a. 



Unio Lessoni, Kiister, Conch. Cab., 1856, 135, pi. 36, f. 4. U. depressus, 

 Lesson, Voy. Coq., ii, 1830, 427, pi. 15, f. 5. U. australis, Kiister, Conch. 

 Cab., 1861, 230, pi. 57, f. 6. Diplodon Lessoni, Kiister: Simpson, Proc. 

 U.S. Nat. Mus., xxii, 1900, 890 ; Suter, T.N.Z.I., xxxvii, 240. 



Shell oblong, obliquely truncated behind, compressed, inequilateral. 

 Beaks strongly eroded in all specimens I have seen. Anterior end 

 short, convex ; posterior end much longer, produced, the dorsal margin 

 slightly sinuate behind the ligament, basal margin lightly rounded, 

 sometimes faintly excavated at the middle. Sculpture : Surface with 

 distant flatly elevated rest-marks, which, together with the intervening 

 space, are covered with very fine threadlike concentric lines ; towards 

 the base and posterior margin the growth-lines are more distinct and 

 slightly foliated ; there is no trace of radial sculpture. Epidermis 

 olive to dark brown. Interior bluish nacre, with yellowish patches 



