ortmann: south American naiades. 459 



coming there into contact in a sharp angle. There are sometimes irregularities 

 in the beak-sculjiturc obscuring their radial character. 



Hinge-teeth alwa3\s present, but rather variable in number, size, and shape. 

 Normallj^ there are, as in most other Naiades, two laterals in the left valve, and 

 one lateral in the right valve, but individual variations may occur in this respect. 

 The pseudo-cardinals arc much more variable. Originally their arrangement 

 seems to be similar to that found in primitive Unionidce: that is to say, there are 

 two in the left valve, and one or three in the right; of the latter, the middle one 

 may be the main tooth, fitting in between the two teeth of the left valve, and the 

 anterior and posterior tooth may ])e accessory. However, this arrangement is 

 very often changed by the suppression of certain teeth, and the addition of others. 

 The most general condition is when of the two pseudocardinals of the left valve 

 the posterior is more or less obsolete, and often entirely wanting, and only the 

 two anterior teeth of the right valve are present. Thus the left valve appears 

 to have only one pseudocardinal {Aspidon of Von Ihering) and two laterals, and 

 the right valve has two pseudocardinals {Dexion and Epidexion), and one lateral. 

 This is chiefly the case in certain species of Diplodon, where the pseudocardinals 

 are greatly compressed, and directed more or less parallel to the hinge-line, and 

 this should be kept in mind for the distinction of those species of Diplodon, which 

 resemble in shape certain North American species of Ellipiio, where the normal 

 arrangement shows two pseudocardinals in the left, and one in the right valve. 



This is not the only variation. Additional teeth may turn up, and chiefly when 

 in the left valve anterior to (or above) the anterior pseudocardinal an additional 

 tooth is developed (Epaspidon, Von Ihering). 



The names Asjndon and Epaspidon, Dexion and Epidexion, may be used to 

 advantage; but we should not forget that originally there is in the left valve a 

 tooth behind the Aspidon (regarded by Von Ihering as being accessory), and that 

 there is often in the right valve another (third) posterior tooth, behind the Dexion.'' 



Further, there is much variability in the shape of the teeth. The pseudo- 

 cardinals, as has been stated, are often compressed, lamellar, and smooth. But 

 often they are more solid, or stumpy, when they are frequently more or less split 

 and divided, sometimes almost cut up into a numl^er of teeth. The laterals may 

 be smooth or corrugated, the corrugations standing obliquely on the faces; and in 

 certain genera there are characteristic regular and parallel striations or ridges 



' Cuiuiwie also Oillincr, Ilils, p. .")74 IT. (Homologies of hiiige-tcctli of " Unionidw"). Odhner 

 seems to be right in a general way, l)u( (lie liingc of the Naiades is extroinely complex ami imliviilually 

 variable. 



