Bulletin 31 32 



DalPs orii^iual descripiion. — Shell small, fiattish, with small, unequal 

 ears and rounded tlisk ; fourteen or fifteen ribs carrying basally three dense- 

 ly finely imbricated, rounded threads, the interspaces narrower with tv.-o 

 crenulate threads; submargins with close, fine, imbricate threads ; ears prom- 

 inent, with a deep wide, byssal notch, radiately imbricate with coarse ele- 

 vated radial threads ; interior with shallow sulci, the cardinal crura devel- 

 oped but no lirse on the disk. Alt. 22, lat. 22 mm. 



This species differs from the Claibornian* P. dcshaycsii Lea by its 

 threaded and less individualized ribs, its similarly sculptured valves, more 

 conspicuous notch, and concentric sculpture and smaller size when adult. 



We have emended the spelling of this species, the "h" having 

 crept into the name from bad orthograpy of the collector. Dall 

 states that this species is "abundant at Waiitiibbee." Our col- 

 lections furnish many fine specimens from Wautubbee, but we 

 scarcely feel like giving it the wide geologic range implied in 

 "Claibornian and Jacksonian of Louisiana, Alabama and Mississ- 

 ippi." 



This species is extremely variable in api^earance, even at the 

 t3'pe locality. The right valve seems to be far more abundant 

 and is flatter than the left and is more apt to assiune a worn ap- 

 pearance (see fig. ij. While the ribs, abotit the periphery in the 

 right valve, nearly always show the tri-partite markings described 

 by Dall, such secondary ornamentation on the left valve are far 

 less conspicuous though present on the lower margin of the shell 

 (see fig. 3). On the type specimen, however, such markings 

 are well defined. 



Whenever the middle one of the three riblets is strongest, 

 then, whether the rib be worn or intact, it apears narrow. When- 

 ever the lateral riblets are well developed the rib has a broad ap- 

 |)earance and the interspaces seem narrow and deep. In extreme 

 cases the riblets are not distinctl}' developed but the imbricate 

 lines of growth are well defined. The general aspect of the shell 

 is accordingl}- greatly changed. Quite often, however, after a 

 very marked growth line, the tri-partite ornamentation reappears. 

 A verj' gibbose, perhaps somewhat pathologic specimen in our 



*Note that on p. 738, Trans. Wag. Ill no Pectens are known from the 

 Claiborne sand. 



