200 



Bulletin 39 



37« 



large shell near the ventral margins; the interspaces are covered 

 with fine, raised, concentric lines, best seen on the umbos or on 

 young shells; the}' become more crowded towards the ventral 

 margins and may pass over the tops of the ribs as well; the dor- 

 sal and posterior submargins are strongly flattened and sculp- 

 tureless; the anterior ear is a little larger, projects forward and 

 carries, in the right valve, a byssal notch; it is sculptured with 

 several radial riblets; the posterior ear has its upper half nearly 

 smooth or but faintly sculptured, but its lower half carries a 

 band of 5 or 6 strong, radial cords. 



Height 29, breadth 34, thickness 10.50 mm (right valve) 

 34 38 23 (both valves) 



39 43 14 (left valve) 



This is the common Pcden in Costa Rica and frequently 

 forms zones in the lower part of the Gatun formation. All our 

 specimens have 21 ribs, which vary from strongly rectangular 

 in section with dee]) interspaces to more flattened and rounded 

 on the larger individuals. 



Bose has described as Pecten santarosanus, a small scallop 

 shell from the Miocene of the State of Oaxaca, Mexico. The 

 ribs number 23 to 25. Later in the same volume, he figured a 

 larger Pecten from the Miocene of La Barranca de Santa Marie 

 Tatella, as Pecten santarosayius. This shell has but 21 ribs and 

 is equivalent to Toula's levicostata. If this larger specimen of 

 Bose's is equivalent to his typical santarosanus, this name must 

 of course take priority over Toula's levicostatus . As Bose's typi- 

 cal santaroscijius has 23 to 25 ribs, v/hile in levicostatus the ribs 

 appear to be constantl}' but 21 in number, they probably repre- 

 sent two different species. 



In the Dominican Miocene levicostatus is represented by the 

 Pecten excentiicus Gabb, which appears to differ constantly by its 

 lower and more rounded ribs and smaller ears. The recent 

 Pacific Pectefi ventricosus Sowerby is closely related and probablj^ 

 the direct descendent of levicostatus. In that species, the ribs 

 are low and rounded as in excentricus and as occasionally seen in 



