NO. 1312. SYNOPSIS OF THE VENERID.E—DALL. 349 



Tropical and subtropical waters of America; receding in time to the 

 Miocene. This group is unique in the family in the peculiar sulcation 

 of the inner margin, which is only paralleled elsewhere once among 

 the Astartida3. A Pacific coast species is viviparous. 



Genus TIVELA Link, 1807. 



Type, Venus corhlcula Gmelin (= V. rtiactroideH Born). 



Shell porcellanous, solid, smooth externally with a dehiscent perios- 

 tracum; the coloration variable with a tendency toward dark brown 

 and purple; valves trigonal, subequilateral, with prominent beaks and 

 a short ligament; lunule large, faintly defined, escutcheon not defined; 

 pallial sinus small, free, rounded in front; hinge variabk^ with anterior 

 laterals and from three to six cardinals, partly rugose and some of 

 whii'h ma}^ be bifid. Habitat, subtropical and tropical seas. 



Section Tivda s. s. Tj^pe, Venus mactroideH Born. 



Valves trigonal, with smooth interior margins, usually a pilose 

 periostracum over a polished surface; cardinals varying in difi'erent 

 species. 



This is Trigona Megerle, 1811, not Jurine, Hynienoptera^ 1807; and 

 perhaps Dollfusia Cossmann, 1886, which 1 know onh^ by figures. 

 The group is unique in the variability and occasional large number of 

 cardinals, which are perhaps due to splitting up of the originally 

 single posterior cardinals. 



Section Pachydesma Conrad, 1854. Type, Donax stultoruin Mawe. 



Shell very large and ponderous, with smooth interior margins and 

 a thick vernicose periostracum; hinge with four cardinals in each valve. 



This is Ti'lgonella Conrad, 1837, not of Da Costa, 1778. It is a 

 Californian type. 



Section Euflrcla Dall, 1891. Type, K. pci'jilr.nt Stearns. Argentina. 



Shell small, elongate-trigonal, with crenulato interior margins, thin, 

 polished periostracum, three left and four right cardinal teeth. 



This t3^pe points the way toward Sunetta. 



Genus SUNETTA Link, 1807. 



Type, Donax scrljjfa Linnaeus. 



Shell variable in form, smooth or concentricalh' sculptured, polished, 

 often with vivid coloration; with an impressed, unequally divided 

 lunule, larger in the right valve, and a deeply excavated escutcheon; 

 posterior end of shell shorter than the anterior; pallial sinus wide, 

 short, and rounded; inner margins conspicuously crenate; three cardi- 

 nals in each valve, and rather elongate anterior laterals. 



Eocene of south Europe and tropical seas of the Old World. 



Section Smiitta s. s. Type, Donax >«-rlj>f(( Lininvus. 



Shell elongate-ovate, more or less inequilateral, the edge of the 

 posterior cardinals finely rugose; sculpture concentrically sulcate or 

 striate. 



