1921] NATURAL SCIENCES OF PHILADELPHIA. 411 



and varies rather widely in the degree of development of the rib- 

 ornamentation. 



In a form of this species which may be called Pecten thetidis pelei 

 (Plate XLI V, figs. 4, 5 ) the ribs have wider summits, more strongly 

 developed spinose flanges, and on the slopes of the ribs there are 

 one or tivo spinose riblets. The submargins and ears have a close 

 sculpture of spinose riblets. Alt. and length 28, semidiam. 11 mm. 



Type no. 2853 A. N. S. P. 



Pecten plurinominis Pils. and Johns. Plate XLV, figs. 1, 2. 



Proc. A. N. S. Phila. 1917, p. 193. 



Pecten scLssuratus Dall. 



Pecten (Aequipecten) scissurahis Dall, Trans. Wagn. Inst, iii, p. 71.5, pi. 34, 

 fig. 4. Brown and Pilsbry, Proc. A. N. S. Phila., 1911, p. 364, pi. 28. figs. 

 2, 5. 



The sculpture is highly developed in this species. 



Pecten interlineatus Gabb. Plate XLV, fig. 3. 



Pecten interlineatus Gabb, Trans. Amer. Philos. Soc, xv, 1873, p. 256. 



On the left valve there are 15 or 16 rounded and not very high 

 major ribs., a riblet running in each interval; close to the base there 

 are two or three in each interval. Anterior submargin with about six 

 riblets, posterior ribs and riblets are granose. The anterior ear 

 has few fine riblets, posterior ear coarse ribs. There is a fine con- 

 centric sculpture of regular hair-lines, but more or less wholly worn 

 from the summits of the ribs. The right valve is a trifle less convex 

 with the same sculpture except that both ears have fine riblets. 

 Four well-developed teeth in the short ctenolium. 



Length 23, alt. 24.7, diam. 7.2 mm. 



Type no. 2856 A. N. S. P. 



Pecten oxygonum Sowerby. Plate XLV, flgs. 4, 5, 6. 



Pecten oxygonum Sowb., Q. J. Geol. Soc, vi, 1849, p. 52. 



A series of 26 valves seems to be referable to Sowerby 's species. 

 Doctor Dall has shown that neither of the specimens now pre- 

 served in the typical tray of Sowerby 's collection agree with the 

 diagnosis. 



The 18 or 19 ribs have either distinctly angular or very narrowly 

 rounded summits. There are fine, thread-like concentric striae. * 

 Submargins plain. The left valve is more convex than the right. 

 There is no ctenolium. 



Length and alt. 26.3, convexity 7.2 mm. (Left valve figured). 



