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64 CONTRIBUTION TO THE PALEONTOLOGY OF TRINIDAD. 



Corbula sp. indet. Plate IX, Figures 26, 27. 



A number of moulds of the interior of a species of Corbula were found in the 

 ferruginous marl south of Pitch Lake. It was evidently a common species. 



In form the shell was of the short, high, very convex type; the moulds are 

 nearly equilateral, rounded anteriorly, truncated posteriorly; with the two valves 

 not differing much in size. 



Length of mould 11, height 9, diameter 7 mm, 



Locality. — Southern main road just south of Pitch Lake, Brighton, Trinidad. 



Geological horizon. — Oligocene. 



Genus PHOLAS (Lister, 1687), Linnaeus, 1758. 

 Pholas mackiana new species. Plate IX, Figure 31. 



Description. — Shell oblong-ovate, slightly inflated, small for the genus, thin 

 and fragile; sculpture of concentric fluted lines of growth which at the anterior end 

 of the valve are raised into close-set, radial riblets on which the fluting is more 

 strongly marked than elsewhere; beyond the anterior end the concentric lines 

 become more pronounced and are marked with V-shaped folds, while the radial 

 sculpture is lost. 



Height of shell 14 mm. 



Remarks. — When compared with the Miocene species of Pholas from the 

 southeastern United States, the Trinidad shell is found first to differ very mark- 

 edly from P. arcuata Conrad. For in the latter species the radial sculpture is 

 prominent all over the shell; and it is also much heavier and less fragile than 

 mackiana. To P. producta Conrad and P. memmingeri Tuomey and Holmes, 

 both South Carolinian species, the shell from Trinidad bears only a generic 

 resemblance. 



On comparing P. mackiana with living species, it is at once seen to be quite 

 unlike the common Pholas costata Linn, that ranges from Massachusetts to 

 Brazil. P. costata is the recent analogue of P. arcuata, and like the latter has 

 radial sculpture all over the shell. But the Trinidad fossil does resemble in type 

 of sculpture both P. campechensis Gmelin living in the southern United States 

 and the West Indies, and P. chiloensis on the shores of Peru and Chile. These 

 species, as Tryon has pointed out, 34 are very closely related and apparently almost 

 identical, which is surprising because of their being east and west coast shells. 



Now, as the writer is convinced, the key to the explanation of this resemblance 

 of these east and west coast recent species is the Trinidad fossil. It represents 

 the ancestral Oligocene stock from which both the recent forms started from a 

 center of development in the lower Antilles. Before the rise of the Isthmus of 

 Panama the western form migrated to the Pacific and has continued to develop 

 there, while the eastern has been slowly spreading along the Atlantic coast. 



Pholas mackiana thus furnishes a further indication of the Oligocene age of 

 the asphaltic marl in which it was found. 



34 Mon. Pholadidae, Proc. Acad. Nat. Sci. Phila., p. 76, 1862. 



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