CONTRIBUTION TO THE PALEONTOLOGY OF TRINIDAD. 39 



be almost perfectly duplicated by shells of the same species collected by Professor 

 Harris from the basal Eocene of Alexander, Arkansas. 10 There can be no ques- 

 tion of their identity. The Soldado convex valve measures in height 22, greatest 

 width 18, greatest thickness 14 mm. 



This species is closely allied to 0, thirsce Gabb, which is the Lignitic analogue 

 of this Midway shell. 



Locality. — Bed No. 2, Soldado Rock, Gulf of Paria. 



Geological horizon. — Midway Eocene. Equivalent to that of Alabama and 

 of the Rio Maria Farinha beds, Brazil. 



Ostrea thalassoklusta new species. Plate VI, Figure 6. 



Description. — Shell small, subcircular in outline, large valve moderately con- 

 vex; hinge margin slightly alate; basal margin sinuous; valve smooth except 

 for a deep, rather broad sulcus which extends from the umbonal region to the 

 basal margin of the shell, rendering the central part of the valve concave. 



Height of larger valve 23, greatest width 21, greatest thickness 10 mm. 



Remarks. — This Ostrea is closely allied to both the Midwayan 0. pulaskensis 

 Harris and to the Lignitic 0. thirsce but it can readily be distinguished from both 

 by its subcircular form, and especially by the characteristic, deep sulcus which 

 takes the place of the umbonal-basal ridge in those species. Where they are 

 convex it is concave. 



Locality. — Bed No. 2, Soldado Rock, Gulf of Paria. 

 Geological horizon. — Midway Eocene. 



Ostrea thirsae Gabb. Plate V, Figures 6, 7, 8. 



Ostrea emarginata Tuomey (name only), 2d Biennial Rept. Geol. Surv. Alabama, p. 269, 1858. 



Gryphcea thirsce Gabb, Proc. Acad. Nat. Sci. Phila., p. 329, 1861. 



Ostrea thirsce Heilprin, 3d Ann. Rept., U. S. Geol. Surv., p. 311, pi. 63, figs. 4-6, 1884. 



Gryphcea thirsce Aldrich, Bull. I, Geol. Surv. Alabama, p. 58, 1886. 



Ostrea thirsce Harris, Bull. Am. Pal., vol. I, pp. 232-233, pi. 6, figs. 5, 6, 1896. 



Ostrea thirsce Dall, Trans. Wagner Inst. Sci., vol. Ill, p. 680, 1898. 



Gabb's original description. — "Rounded, sub-triangular. Lower valve; beak 

 very small, and close to the hinge, never exsert. Umbone rounded, very promi- 

 nent and somewhat compressed laterally, the rounded elevation continuing more 

 or less regularly, becoming broader to the middle of the basal margin, at which 

 point this margin is always somewhat emarginate. Ligament area broad, tri- 

 angular, transversely striate, and with a slight irregular depression in the middle. 

 Interior of valve very deep. Muscular impression nearly ovoid, narrowest on 

 the inner side. External surface marked by a few small, irregular, squamose 

 ridges, most numerous and distinct directly behind the emargination of the ba<e. 

 Upper valve unknown. 



"The species resembles, remotely, some of the narrower forms of G. vesicularis 

 Lam., but after comparing the series before me with numerous authentic speci- 

 mens of that species, both American and European, some of the latter labelled 



10 Cornell Paleontological Museum, No. 8400. 



