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617 



TERTIARY KAUNA OF R.ORIOA ' 



Shell cquivalve, inequilateral ; the beaks anterior, opisthogyrate, rather 

 adjacent ; ligament transversely grooved, lozenge shaped, not occupying the 

 whole cardinal area ; posterior slope of the valves usually longer, subtruncate, 

 or bounded on either side by an umbonal keel or ray; tooth-line rather long, 

 terminal teeth often A-shaped. 



This group is American and Indo-Pacific in its recent distribution ; the 

 known fossils are all American. Besides those enumerated in this paper, 

 Guppy has described from Trinidad Area trinitaria (Manzanilla) and A. ccti- 

 trota (Matura), both of which belong to this group. The former might pass 

 for an ancestor of the (now Pacific) A. rcversa, while A. centrota, probably a 

 Miocene species, offers a diminutive facsimile of A. limiila. Area trapezia 

 Desh., from West Mexico, and A. Martinii Recluz, 1852, from Santa Caterina, 

 15ra7.il, belong to this group. The latter is probably the shell living in the 

 Gulf of Paria, which Guppy has referred to his A. centrota. They are very 

 similar, and the living shell may perhaps be the unfigured A. bisitlcata of 

 I .amarck. The name Area Martinii is preoccupied by Bolten, and must be 

 dropped. A. hemicardium Koch, as figured by Philippi, also belongs here. 



This subgenus is intermediate between the typical Arks and Scapliarca. 

 The original type has the anterior side of the shell longer and the posterior 

 side short and truncate, but in N. ponderosa the two sides of the hinge are 

 subequal, and in the fossils the posterior side is much longer than the ante- 

 rior. The definition in the text-books, drawn from A. reversa, must therefore 

 be materially modified to be true of the very natural group to which it belongs. 



Subgenus SCAPHARCA (Gray) Dall. 



Scapliarca dray, P. Z. S., 1847, p. 206; H. and A. Adams, Gen. Rec. Moll., ii., p. 537, 

 1858. Type, ./. incquii'ali'is Brug. 



It is a matter of some difficulty to decide which of the various names 

 bestowed at various times by Gray should be regarded as most inclusive and 

 predominate in the necessary consolidation of the group. The names Scnilia, 

 Argina, Luimrca (and, according to Agassiz, Noctia, though Gray does not 

 mention it in his list of 1847) appeared first without diagnosis or figures in 

 the little manual entitled "Synopsis of the Contents of the British Museum," 

 prepared for the use of visitors to that institution. According to the rules of 

 nomenclature, these names were not fully established either by their nude in- 

 sertion in this list or in that of the synonymy of 1847. The latter contained, 

 in addition, Anaiiara Gray for A. antiquata and Scapkarca for A. incquivalns. 



