516 PROCEEDINGS OF THE ACADEMY OF [Dec, 



So far as exposed, this fossil resembles the Costa Rican form, 

 but its condition does not admit a positive identification. As 

 this form is intimately related to C. reevei Gabb, of Santo Domingo, 

 wc subjoin some account of that species. 



Cardium (Trachycardium) durun n. sp. PI. XXIII, fig. 6. 



This species is represented by a cast of the right valve with the 

 inner layers of shell adhering. It is strongly convex with prominent, 

 very full beaks. Twenty-five strong, angular ribs are indicated, 

 those of the posterior margin terminate in teeth. There is some 

 indication that the ribs had lateral ridges. The posterior slope 

 shows a wide, shallow radial concavity. 



Length 39, alt. 41.5, semidiam. 19 mm. 



Gatun Formation at the Spillway. 



This is a longer shell than C. stiriatum B. and P., the valve- 

 margins being not far from circular, while in C. stiriatum the outline 

 is conspicuously oblong. 



Dosinia delicatissima n. sp. PI. XXVI, fig. 1. 



Dosinia (Artemis) cf. Acetabulum Com 1 ., Toula, I.e., p. 727, pi. 27, figs. 8, So. 

 Specimens from the Spillway agree well with Dall's account and 

 figures of D. liogona Dall (Trans. Wagner Inst., Ill, p. 1230, pi. 53, 

 figs. 4, 7; pi. 54, fig. 11), except that the shell is remarkably thin for 

 a bivalve of this genus. A valve at least 60 mm. in length is only 

 1.3 mm. thick in the thickest part. Moreover, the sculpture does 

 not rise in "sharp fine lamellae towards the ends of the shell," as 

 described for that species. Towards the ends of the shell the con- 

 centric ridges between the grooves are more raised than in the middle 

 of the valves, but they are rather too thick and blunt to be called 

 lamellae. The specimens are all in poor condition so that the 

 sculpture of the beaks is a little in doubt, but from a small area 

 exposed in one specimen the very young shell would appear to be 

 smooth. 



Length 48, alt. 46, semidiameter about 11 mm. Less perfect 

 specimens than that figured are larger, up to 55 to 60 mm., or even 

 more. 



Very plentiful in the Gatum Formation at the Spillway. 



It occurs also in the Pecten bed at Tower N. 



The Miocene D. acetabulum Conr. is a decidedly more solid shell,' 

 noticeably differing from D. delicatissima in sculpture. 



Petricola millestriata n. sp. Pi. XXVI, fig. 2. 



The shell is short, the height contained about 1^ times in the length. 



