570 



MEMOIRS OF THE CARNEGIE MUSEUM. 



Distribution. — Known only from the uppor Rio Paraguay-. Simpson also 

 gives "San Paulo, Brazil," but I do not know upon what authority. 



My specimens agree well with Von Ihering's description and figure, l)ut they 

 are larger. Von Ihering says that the color of the epidermis is dark olive or black- 

 ish. In our largest and smallest (half-shell) specimens the surface is much corroded 

 and worn, and brownish green. In the specimen of medium size, which is better 

 preserved, it is yellowish green, with a few scattered dark green spots upon the disk, 

 becoming more frequent near the beaks. Upon the posterior ridge, there are a 

 few fine, interrupted, dark green rays, and a broader ray runs over the posterior 

 slope. 



In the two smaller specimens, the structure of the hinge corresponds entirely 

 with Von Ihering's description. In the largest, however, the anterior tooth of 

 the left valve is obsolete, and so is the groove between the two teeth. The pos- 

 terior tooth is broad and slightly bifid. In consequence of this the groove of the 

 right valve is also broader and has a radial ridge in its bottom, and the anterior 

 tooth of the right valve is very small. I consider this an abnormality. In this 

 specimen there apparently is ordy one (bifid) tooth in the left valve, and two small 

 ones in the right, with an accessory ridge in the groove between them. 



In all three of my specimens, the ligamental sinus is wider and shallower than 

 figured by Von Ihering, which may be due to their greater age. 



Measurements. 



The measurements given by Simpson (1914, p. 1395) are entirelj^ wrong, and 

 are those of Fossula halzani Von Ihering. 



Genus Fossula Lea (1870). 

 Lea, Syn. 1870, p. 72,- foot-note 1; Von Ihering, 1893, p. 62; Simpson, 1900, p. 914; 



1914, p. 1396. 



The distinguishing character of this genus is found in the hinge-teeth, of which 

 there are two in the right valve, enclosing between them one in the left valve. The 

 hinge-plate is narrow, the teeth are stumpy (not vertically compressed or spoon-like 

 as in Monocondylcea). There are traces of additional teeth and irregularities of 

 the hinge, which, however, are variable. The "cement processes" described by 

 Von Ihering are not always present. 



