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637 



TERTIARY FAUNA OF FLORIDA 



This species has always been rather rare, and has been confounded with 

 its undoubted descendant, the Area floridana of Conrad, found living in 

 Florida waters. Nevertheless, the recent and the fossil shells are readily dis- 

 tinguished on comparison. It is probable that the rarity of the living shell 

 has prevented the comparisons being made. This species had previously 

 been named sccticostata by Reeve, from a specimen of which the habitat was 

 unknown. This name, of course, will have to be adopted. 



In A. licnosa there are about forty ribs in a specimen one hundred and 

 eight millimetres long; these ribs are deeply grooved down the centre, and 

 the ridges on either side of the grooves are likewise longitudinally grooved 

 with one or two incised lines. The interspaces between the ribs are narrower 

 than the ribs ; the beaks are less anterior than in A. sccticostata. In the latter 

 the ribs are much narrower than their interspaces, flat-topped, and distally for 

 a little more than half their length in the adult the top of the rib has a broad, 

 shallow channel. In no case are there any subsidiary grooves. Minute con- 

 centric ridges are quite obvious in both species, but the fossil has the ridges 

 more generally and conspicuously beaded. In other respects the shells are 

 extremely similar. 



Scapharca (Scapharca) hypomela n. s. 

 PLATE 33, FIGURE i. 



Oligocene of the Ballast Point silex beds, Tampa Bay, of the lower bed 

 at Alum Bluff, and of the Chipola marl, Chipola River, Florida. 



Shell of moderate size, long, inflated, with rather low, mesially com- 

 pressed, prosogyrate beaks ; left valve with about forty-three deeply chan- 

 nelled, flat-topped ribs with fine, regular, concentric beading, except on the 

 posterior slope, where the ribs are lower, flatter, and obsoletely channelled ; 

 near the margin some of the ribs have a second set of finer grooves ; hinge- 

 line straight, anterior end descending vertically, then obliquely rounded into 

 the base, which is nearly parallel with the hinge-line; the posterior end 

 descends more obliquely and the basal angle is prolonged a little and 

 rounded ; the interspaces between the ribs in both valves are very narrow, 

 and on the right valve the beading is less conspicuous ; the cardinal area is 

 somewhat concave, flattish, with three or four concentric grooves in loxenge 

 form ; teeth of the hinge similar, numerous, not interrupted, short, vertical, 

 the distal teeth a little longer and more oblique ; margin of the valves fluted, 

 the right valve slightly smaller than the other. Lon. 50, alt. 25, diam. 

 2O mm. 



