24S ON" THE TOPOGHiAPllY AND GEOLOGY 



NEiERA. Gray. . , 



N. alter 11 at a. D-Orb., sp. . '"^ 



Sphena, id. U'Orb., La Sagra's, Cuba, pi. 27, fig. 17-20. . 



Nccerea, id. 11. and A. Ad. Genera Rec. Moll., p. 3G9. _ , 



K". oriiatissima. D'Orb., sp. . . 



SpJienu, id. D'Orb. loc. cit., pi. 17, fig. 13-10. 

 Necera, id. 11. and A. Ad., Gen. p. 3G9. 



Despite the difterenees in D'Orbigiiy's figures quoted, I am hardly prepared to 

 admit the validity of the two species. My specimens certainly run into each other, 

 so that although I can lay out the typical forms, it is not so easy to draw the line 

 across the 'gradation. The more common form is one fi.nely costate behind and 

 with one or two large ribs in advance. 



PANDORA. Brug. 



P. i neons pi cua. Gabb. n. 9. 



Shell small, inequivalve, very inequilateral ; beaks small, about one-fourth of the length from the anterior end ; 

 cardinal margin gently sloping, straight behind; base broadly convex. Right valve convex, with a sharp angular ridge 

 running posteriorly from the beak, and sometimes a faint groove above it. Left valve concave, angulated on the 

 X)Osterior cardinal margin. Length .3 inch, vi^idth one inch. 



The smallest species of the genus with which I am acquainted. It is almost 

 always found in a denuded or otherwise injured condition, where the other fossils 

 are remarkable for the beanty of their preservation. It belongs to the true genus 

 Pandora as restricted by Carpenter. • * 



MACTRELLA. Gray. ' " . ' 



M. alata. Spengl., sp. ' • ■ 



Mactra alata. Si3eng].,,Skriv. Nat., Vol. V., part 2, p. 99. ;. . 



Mactrella, id. Gray, H. and A. Ad. 



A few small specimens which agree perfectly with recent shells in the Academy's 

 collection. It is stated by Reeve to be a West Columbian species. 



RAETA. Gray. 



R. canaliculata. Say. sp. 



Lutraria canaliculata. Say., Jour. Phil. Acad., Vol. IL, p. 311. ' • 



Baeta, id. Gray, Adams. ■ • 



A single half-grown specimen, which seems rather broader posteriorly than is 

 common in this species. 



TELLINA. Linn. 

 T. (Peronffioderma) punicea. Born, p. 33, pi. 2, fig. 2. 



A common shell on both sides of the Isthmus. My specimens are broader and a 

 little more rounded posteriorly than the recent form. It is possible that they may 

 constitute a distinct species. They agree in size and sculpture. ^ 



T. (P.) alternata. Say., Jour. Phil. Acad., 2 ser.. Vol. IL, p. 274. \. ' ■ 



Agrees perfectly in form, size and sculpture with the recent shell. 



