- 140 - 



defined. The ribs - four to six in number - in the suloiis are nar- 

 rower and flatter than the others; this is true also of the ribs 

 that have a corresponding position on the left valve. On imworn 

 valves fine, evenly and closely- spaced concentric lines, usually 

 more prominent in loft valves, appear in the intercostal spaces; 

 the median teeth are short and vertical, vshile the anterior and 

 posterior ones are long-er and slig-htly oblique. The deep, short 

 crenulations of the internal nnrgin are obsolete in the melian vent- 

 ral region, i. e., in the region of the external median sulcus. 



Dimensions .- Right vnlve: Lenp:th, 9 mm.; alt. 6 mm.; semi- 

 diam. , 2.5 mm.; length of hinge 5 ram. left valve: length, 9 ram.; 

 alt., 5.2 mm.; semidiam., 2.5 mm.; length of hinge, 1-.8 mm. 



Remarks .- This is a small form, but there is no doubt that 

 it actually is adult, since it is r.'ell re-nresented by an amazing 

 n^imber of individuals of various staR^es of grov;th. Because of its 

 shape and relatively high urabones it is sharply set off from the 

 ; oui g of associated Scapharcas. 



Occurrence .- Lower Kioeene: 3ov/den beds, Bowden, Jamaica. 

 (Dall, 1898). 



