1912.] NATURAL SCIENCES OF PHILADELPHIA. 511 



are a little flattened on the summits, have very steeply sloping 

 sides, and are parted by intervals decidedly narrower than the ribs. 

 Over all there is a fine concentric sculpture of delicate lamina?, 

 which remain much more prominent in the intercostal spaces. In 

 the right valve the ribs are noticeably narrower, about equal to their 

 intervals, and they are almost obsolete in a rather wide band at the 

 anterior end. The anterior ear is ribless. The concentric sculp- 

 ture is less developed than in the convex valve. The height is 

 52 mm. in the largest co-type — a right valve. 



Gatun Formation at the Spillway. 



This scallop may be readily recognized, even in fragments, by its 

 finer sculpture, which is unlike other Isthmian species. 



Pecten oxygonum oanalis n. subsp. PI. XXIII, fig. 3. 



We refer to this subspecies a series of scallops from the Pecten 

 bed at Tower N, Culebra Cut, Canal Zone, which agree with the 

 Costa Rican P. o. optimum in the main, but differ by having the 

 radial striae almost equal, without an enlarged one on the ridge 

 of each rib. The number of ribs is the same as in optimum. The 

 figured specimen measures, alt. 45.5, length 44 mm. Some are 

 larger, up to 55 mm. in length. 



This is a very abundant and characteristic fossil of the upper 

 bed of hard limestone near Tower N. The shells cannot be extri- 

 cated from the rock, in which they are exposed by breaking it up. 

 While this Pecten is obviously close to Costa Rican and Haitian 

 forms, it seems advisable to signalize the minor differences of the 

 races by subspecific names. 



Pecten oxygonum optimum n. subsp. PI. XXIII, fig. 2. 



Pecten paranensis d'Orb., Gabb, Journ. A. N. S. Phila., VIII, p. 347. pi. 

 45, fig. 24. Not of d'Orbigny, Voyage dans l'Am&\ Merid., Pakean- 

 tologie, p. 132. 



The shell is larger than P. oxygonum Sowb., of the Santo Domingo 

 Oligocene, with lower ribs. There are at least 19 ribs, several 

 at each end, narrow, low, and slender, the rest rounded, broad, and 

 rather low, parted by somewhat narrower intervals. Both ribs 

 and intervals bear unequal radial stria? or threads, of which one on 

 the summit of each rib is somewhat larger, giving the ribs a cari- 

 nated appearance. There is also an enlarged thread in the middle 

 of some of the intercostal valleys. About 11 stria? may be counted 

 between the summit of one rib and the next. Auricles strongly 

 striated radially. The whole surface (except near the beaks) bears 



