io8 Studies in Geology, No. 3 



Area (Scapharca) donacia DalP"^ is in some ways more 

 similar than is cibaoica. In general contour and sculpture 

 it offers a strong superficial resemblance, and Woodring ^"^ 

 notes the presence of a similar sulcus on the umbo of the 

 right valve of forms from the Bowden beds. Comparison 

 with Woodring's specimens shows the sulcus to be shallower 

 and longer in donacia than in vanholsti. In the left valve 

 of donacia the sulcus is absent or very poorly developed : 

 in vanholsti it is equally prominent in both valves. A further 

 difference lies in the extent of the indentation, which in 

 donacia broadens toward the ventral margin, causing a 

 slight but perceptible emargination of the median ventral 

 margin ; in vanholsli, as noted above, the sulcus is confined 

 to the immediate surface of the beaks, and is totally lost 

 on the main surface of the shell, with no trace of indenta- 

 tion of the ventral margin. Thus, in donacia the develop- 

 ment of the submedian sulcus appears to be at a stage mid- 

 way between that of cibaoica and vanholsti. 



The most striking difference between vanholsti and donacia 

 is in size, vanholsti being decidedly larger. Other distinc- 

 tive differences are : the ribs of vanholsti are more promin- 

 ent and sharply chiselled, showing, as well as the interspaces, 

 the concentric growth-crenulations absent in the ribs of 

 donacia; the angle between the anterior margin and the 

 hinge-line, obtuse in donacia^ is about 90° in vanholsti; and 

 the beaks oi donacia are much more prominent than those 

 of vanholsti. 



Lower Zorritos (base). Escarpment overlooking Lobitos 

 well in quebrada north of Quebrada Boca Pan. 



Trans. Wag. Inst, vol. 3, pt. 4, p. 649, pi. 33. % U, i^ 

 The Mollusca of the Bowden Beds of Jamaica, Mss. 



