part 8] THE TKIAS OF NEW ZEALAND. 203 



in this respect that of the Australian Permo- Carboniferous genus 

 Eurydesina. The hinge seems to be quite edentulous, and the 

 articulation was so feeble that the two valves are alwaj's discon- 

 nected; but I have a few specimens in which the valves are only 

 displaced slightly, and they obviously belonged together. From 

 these it is evident that the valves were approximately equal in 

 size and degi'ee of inflation, although whether the valves were 

 exactly equal or not is not clear. I am inclined to think that the 

 left valve may have been slightly smaller than the right. 



The hinge-line is straight or very slightly curved, and is less 

 than the greatest width of the shell. Behind the beak in both 

 valves is a longitudinal, concave, areal furrow, which extends for 

 ^bout two-thirds the length of the hinge-margin, and seems to 

 Jiave accommodated a partl}"^ sunken ligament. There is a small, 

 blunt, thickened septum in the interior of the apex of each A''alve, 

 recalling that in JShjalina or Dreissensia. Behind the beaks 

 is a flattened, obtusely-angular, posterior wing, which in some 

 examples is but siightlv differentiated from the hinder margin of 

 the shell. 



The anterior portion of the shell in front of, or below, the beak 

 differs markedly in the right and left valves, and shows features 

 which are common to all the species of the genus whatever may 

 be their shaj^e or degree of inflation. 



The right valve immediately below the beak bears a strong curved 

 •or semicircular truncation, which is bounded on the posterior 

 ^ide by a rounded, raised or thickened ridge of shell. In front of 

 this truncation, and just below the apex, is a rather long tongue- 

 shaped projection (Pi. XXII, figs. 1, 2 a, & 2 b), and immediately 

 below this is a conspicuous retreat of the shell-margin forming 

 a deep and marked sinus. The upper edge of the anterior tongue- 

 like shelly process carries a raised or thickened ridge. 



In the left valve no concave truncation is seen in front of the 

 beak as in the right valve ; but below the beak the shell-margin 

 is arched and thickened, and retreats so as to produce a narrow 

 byssal opening similar to that of Mt/tilus or 2LjaVina (PI. XX, 

 iig. 5 5). 



The tongue-like process of the right valve projects beyond the 

 plane of junction of the valves, and must have covered the upper 

 part of the opening produced by the retreat of the margin of the 

 -opposing left valve. The opening which remained beneath the 

 process Avhen the two valves were together seems to have allowed 

 i;he passage of a byssus. 



In one example (PI. XXII, fig. 2 «) a fairly-large posterior 

 muscle-scar is observed rather more than halfway down the shell 

 between the median line and the posterior margin ; but I could see 

 no trace of an anterior scar, and in nearly all the specimens the 

 muscle-impressions are quite obscure. One cannot therefore say 

 with certainty whether the genus is monomyarian or diniyarian. 



Dimensions. — The specimens are extremely variable in size, 

 and this feature, together with the shape, will be noticed in the 

 specific descriptions. 



