CONTRIBUTION TO THE PALEONTOLOGY OF TRINIDAD. 61 



For this reason the specimen has been identified as Venerupis with some hesi- 

 tation; but its resemblance to Fischer's figure of Venerupis exotica, and to 

 Kobelt's figure of V. irus (the geno-type) is so striking that although, unfor- 

 tunately, the hinge characters are not known, yet it cannot be any other genus. 

 It does not resemble the Petricolas. 



Apparently the genus Venerupis became extinct on the east coast of the 

 Americas at the close of the Lignitic, just as Cymia died out at the close of the 

 Oligocene. Venerupis is found in the California Pliocene and is now living on 

 the coast of that state. 



The oldest species of the genus are found in the Cretaceous. 



Locality.— Bed No. 8, Soldado Rock, Gulf of Paria. 



Geological horizon. — Lignitic Eocene. 



Genus MACTRA Linnaeus, 1758. 

 Mactra austeniana new species. Plate IX, Figures 22, 23. 



Description. — Shell of moderate size; triangular, rather thin, moderately con- 

 vex; beaks low, approximate; surface sculptured only by concentric lines of 

 growth; pallial sinus small, rounded; hinge of right valve with a short double 

 anterior lateral tooth, a strong bifid or reversed V-shaped cardinal, a triangular 

 chondrophore (about equalling in size the bifid tooth), and a double elongated 

 posterior lateral tooth; the narrow, lanceolate ligament area is separated from 

 the chondrophore by a shelly plate. 



Length of largest specimen 27, height 21, diameter 14 mm. 



Remarks. — Small individuals of this species superficially recall Mulinia later- 

 alis in their general outline; but the hinge characters are very different, — the 

 shelly plate lying between the ligament and the chondrophore separating this 

 shell at once from Mulinia. 



This species seems to be more closely allied to true Mactra, sensu stricto, than 

 to any of the other genera of the sub-family MactrinaB. 



We are enabled by this fact to add a further indication of the Oligocene age of 

 the horizon. For according to Dr. Dall 33 typical Mactras are relatively modern 

 and are not known in America from rocks older than the Oligocene. 



Two suggestions are also given by this shell that the horizon it occurs in is 

 late Oligocene. First, Mactra, sensu stricto, does not occur on the Pacific coast 

 in the recent fauna, although there is one species still living in the Caribbean 

 area. This suggests that the genus developed late in the Oligocene and its 

 spreading westward was interrupted by the rise of the Isthmus. Second, there 

 is also a true Mactra found in the Chipola marls (late Oligocene) of Florida, 

 Mactra chipolana Dall. 



The writer takes great pleasure in naming this shell in honor of Mr. Austen, 

 Librarian of Cornell University. His kind help in matters of bibliography has 

 greatly facilitated our researches. 



33 Trans. Wagner Inst. Science, vol. Ill, p. 892. 



