410 PROCEEDINGS OF THE ACADEMY OF [PaRT II 



shell \Yas not collected by Gabb personally, and nothing is known of 

 its stratigraphic position. 



Doctor Dall has recorded Pecten cadaceus Dall from "Tertiary 

 of Santo Domingo, Gabb"; but the species is not represented in 

 this collection. The same is true of Pecten {Plagioctenium) gabbi 

 Dall. 



Pecten soror (Gab))) Plate XLIV, flgs. 1, 2. 



Janira soror Gabb, Trans. Amer. Philos. Soc. xv, 1873., p. 257. 



Gabb 's description was from right and left valves of two individ- 

 uals. His measurement is from the concave valve catalogued under 

 no. 2862, which may be considered type. It has about 19 rounded 

 ribs (not counting a few very faint ones on the submargins) parted 

 by wider intervals, the whole elegantly sculptured with close, reg- 

 ular and dehcate concentric laminae (fig. 2). Alt. 47 mm. 



The convex valve has 23 strong ribs, slightly wider than their in- 

 tervals, somewhat flattened, and with similar concentric sculpture. 



Length 42.5, alt. 40.5, convexity 14.5 mm. 



There are also valves of several smaller individuals showing the 

 same characters. 



Pecten thetidis Sowerby. Plate XLIV, flgs. 3, 6. 



Pecten thetidis Sowb., Q. J. Geol. Soc, vi, 1849, p. 52. Not Pecten. (Aequi- 



pecten) thetidis Sowerby, Dall, Trans. Wagn. Inst, iii, p. 714. 

 Pecten eugraminaticus Dall, Trans. Wagn. Inst, iii, p. 712, pi. 34, fig. 2. 



The characteristic sculpture of this species has been well though 

 briefly described by Sowerby, in terms which preclude the identifi- 

 cation of thetidis with a species of the exasperatus group, by Guppy 

 and Dall. 



The shell is rather convex, slightly inequilateral, with 21 ribs 

 separated by V-shaped furrows. Except near the ends the ribs are 

 square-topped, having flat or furrowed summits and more or less 

 flange-like edges bearing unequal asperities. There is a beautiful 

 sculpture of fine, nearly regular concentric threads over the furrows 

 and slopes of the ribs. Submargins nearly or quite ribless. Ears 

 are small, and have about 5 slender ribs. No ctenolium. 



Length and alt. 24 mm. 



The species here defined and figured is the only one in the large 

 series which agrees with the Sowerby diagnosis. Gabb's collec- 

 tion covered the ground where the Heneken shells were obtained, and 

 contains practically all of the Heneken species, so that it may fairly 

 be inferred that the only species fulfilling the requirements of the 

 original diagnosis is the true thetidis. It is not an uncommon shell, 



