1902.] VON IHERING— ^[OLLUSCAN FAUNA OF PATAGONIA. 135 



margin is concave below the posterior auricle, then becoming 

 convex, passes gradually into the arched ventral margin ; the 

 ventral extremity is obliquely produced and convex. The outer 

 surface is provided with numerous concentric ribs, which are three 

 to four millimetres high and slightly reflected along the free 

 margin ; the number of these lamellae amounts to eighteen in the 

 ventral half of the shell ; the upper or dorsal half is somewhat 

 defective in the central part. The distance between the ribs is, in 

 general, equal to their height, but there is some irregularity in their 

 size and spacing. Between the ribs the surface of the shell is 

 smooth. 



The ligamental area, which is thirty millimetres in length, is 

 destroyed in the central part, as is also the umbo, the situation of 

 which must have been nearly central. The posterior auricle is 

 small, but well developed ; the anterior is broken away, but must 

 have been much larger than the posterior. The lateral remnants of 

 the ligamental area are strong, eight millimetres broad and 

 obliquely striated by narrow grooves, separated by small ribs, which 

 are the direct continuation of the ribs of the outer surface. It is a 

 remarkable fact that the concentric ribs of the outer surface do not 

 converge toward the umbo, but toward the dorsal margin of the 

 ligamentous area. The inside of the shell shows the simple pallial 

 line, which is distant seven millimetres from the anterior lateral 

 margin and twenty millimetres from the ventral margin. Behind 

 the posterior auricle the shell slopes abruptly toward the margin, 

 while at the anterior margin the transition is gradual. It is impos- 

 sible to recognize the muscular impressions. 



Measui^ements. 



Length, from anterior auricle to posterior ventral 



extremity 95 mm. 



Breadih 60 " 



Diameter of half-shell 22 " 



Another specimen, represented only by the ventral extremity, 

 must have had a length of at least 150 millimetres. 



Locality : Golfo de San Jorge, east of Punta Nova. 



Formation : Lower part of the Patagonian. 



It is not easy to define the systematic position of this species, 

 because the umbo and the central part of the ligamentous area are 

 wanting. The multivincular ligament, the oblique-ovate shape and 



