1240 



a raised thread borders the dorsal margin at the side of the ligament; valves 

 moderately convex, the posterior slightly more attenuated than the anterior end, 

 base prominently arcuate; hinge hardly differing from that of G. hydana, but 

 the pallial sinus wider and more rounded in front. Length of adult valve, about 

 38 mm., of the younger but better preserved valve figured 15, height 12.5, 

 diameter 8.0 mm. ; diameter of adult 20.0, height 32.0 mm. 



This species is easily distinguished from G. hydana by its more equilateral, 

 trigonal form, smoother surface, and more ample sinus. 



Genus TBANSENNELLA Dall. 



Transennella Dall, Proc. U. S. Nat. Mus., vi., p. 340, 1883 ; Fischer, Man. de Conchy 1., 

 p. 1080, 1887 ; Dall, Bull. U. S. Nat. Mus., No. 37, p. 56, 1889 ; Rep. Porto Rico Moll., 

 p. 486, 1901 ; Proc. U. S. Nat. Mus., xxix., p. 509, pi. xxxi., figs. 5-7, 1902. 



Shell small, trigonal, with lively coloration ; smooth and polished or concen- 

 trically striate; hinge with three cardinals in each valve, the middle left 

 cardinal bifid; an elongated anterior lateral in the left valve, received in a 

 sulcus in the right valve; lunule defined by an incised line, escutcheon not 

 defined ; nymphs without rugosities ; pallial sinus angular, free below, obliquely 

 ascending; internal margins of the valves sharply tangentially grooved with 

 numerous sulci. Type T. conradina Dall. Florida. 



This group makes its first appearance in the Oligocene, since which it has 

 been a characteristic American type. The well-developed typical species, so far 

 as yet known, are all members of the Atlantic fauna. No indication of vivi- 

 parity has been detected in them. On the Pacific coast, however, a species 

 which is known to be viviparous, the Venus tantilla of Gould ( Venus rhysomia 

 Gabb), occurs Pleistocene and recent on the coast of California. The charac- 

 teristic sulcations of the margin are, however, feebly developed in the Pacific 

 shell, and further study may perhaps oblige us to separate it from the Atlantic 

 group. At present the fact may be noted that Transennella tantilla represents 

 in the Meretricine line the genus Psephidia of the Venerine line, the latter 

 being, as far as yet discovered, a purely Pacific type. 



The recent Atlantic species of Transennella are T. cubaniana Orbigny, T. 

 conradina Dall, T. culebrana Dall and Simpson, and T. stimpsoni Dall. 



Transennella utica n. sp. 



PLATE 57, FIGURE 12. 



Oligocene of the Chipola horizon at Alum Bluff, Florida, and on the 

 Chipola River; Dall and Burns. 



