GASTROPODS, SCAPI{OPODS, P ELECYPODS: NJC'ULIDAE TO MALLE7DAE


513


 The illustrated left valve and a fragment of the posterior ventral part Of at smaller left valve were collected at the same locality as Anadara dolaticosta. In addition, the collection includes two fragments of Grandiarca too incomplete for identification. All these specimens of Grandiarca are battered and corroded. Their condition stands in marked contrast to the 14 well-preserved marine species with which they are associated.
 Noding of median ribs and distinct extension of the ventral part of the posterior margin is more conunon in A. grandi8 patricia than in A. grandi8 grandis, although better preservation is needed for satisfactory identification. Perhaps the assumed habitat influenced the identification.
 Area patricia, described, but not illustrated, by Sowerby, -was the first of the g'ranI?8 -like fossils to be named. In 1925 the late L. R. Cox, of the British Museum (Natural History) forwarded a cast of the alleged type, actually lectotype, of that species (12828 of the Register of Geological Society of London Collection). It is a right valve of a much-named small Anadara of the subgenus Rasia, named Area pennelli by Gabb. (See p. 506 of present account.) Though it was naive to suppose that this is Sowerby's species, I accepted it (Woodring, 1925a). Sowerby was an experienced conchologst. He compared his species with Area granadis and he named it Area patricia. There is nothing patrician about Area penvelli. For these reasons Weisbord (1929, p. 7) was skeptical about the alleged lectotype, and Rutsch (1942, p. 215) rejected it. On the contrary, Anderson (1,929, p. 149) proposed a new name, Area patriareha, for the traditional Area patricia.
 Charles Davies Sherborn, thle meticulous comnpiler of the Index Animaliumn, registered the Heneken collection, which originally included Area patricia, after it was transferred from the Geological Society of London to the British Museum (Natural History). On page 385 of the original of the Register of the Geological Society of London Collection, Foreign Series, hie wrote the following warning, published in German translation by Pflug (1961, p. 8) :
 Sowerby certainly drew from these specimens; none of
 the types were marked; the loose papers have been shifted about; identification. has in many cases been difficult, S0 the searcher must in every case verify my conclusions and
 accept or reject as hie thinks proper.
 It seems strange that Sherborn identified the small species as Area patricia, alth~oughi, as a. matter of fact, he added two question marks in the Register.
 Two left valves of A rea patricia are in the collections of Caribbean fossils in the British Museum (Natural History) (98047 in Register 19 of Additions to De-


partment, of Paleontology, entered in 1879). A strip of paper glued to the better of the two reads "Miocene, St. Domingo." The Register entry reads "8 bivalves, Miocene, St. Domingo, Col. Heniker [Heneken] Collection." Unless grounds are apparent for doubting the authenticity of the Register data, this lot qualifies as the type lot, or perhaps what is left of the type lot. The better of the two valves has a length of 12-4 mm, the other, which is much worn, is a little larger, 133 mm.
 According to Col. Heneken's account (1853, p. 128), he found "large Conchifera of the family of Arcacea, in great abundance; they cover the surface in patches of compact seams" on "the sandstone plain of Savaneta [Sabanetal ," on the south side of Rio Yaque del Norte between Rio Cana and Rio Guayubin. The Maury expedition collected the large arcid on Rio Cana (Maury, 1917a, p. 25) from strata that are younger than the early middle Miocene Cercado formation farther upstream. Their relations to the late middle Miocene Gurabo formation are unknown, but they presumably were deposited during~ the regressive stage of the transgression that started with the early Miocene Baitoa formation and continued during the middle Miocene, and probably are of late, Miocene age.
 The only USNM Dominican Republic specimens are in two lots collected in the foothills of the Cordillera Septentrional, on the north side of Rio Yaque, del Norte.
 Occurrence: Middle part of Gatun formation (middle Miocene) western area, locality 170.
 Subgenus Potiarca Iredale
iredale, Great Barrier Reef expedition, 1928-29, Scientific Reports, v. 5, no. 6, Mollusca, pt. 1, p. 284, British Museum
 (Natural History), 1.939.
Type (orthotype) :Potiarca (pilula) saccutla Iredale; that is, a
 subspecies of Arca pilula Reeve, living-, western Pacific
 Ocean.
 Ever since Dall's treatment in 1898 (Dall, 1890-1903, p. 633-636, 1898) the American fossil and living species of Potiarea have been assigned to his subgenus Cumearea, the next subgenus under consideration. I am indebted to my colleague, Druid Wilson for pointing out that Potiarca is available for American species as well as western Pacific species. To be sure, Potiarea and Cunearca are more or less similar in several features. Both are inequivalve, the sculpture of the two valves of both is discrepant, and the ribs of both, with the exception of two Miocene species of Potiarea, are undivided. The outline of both also is similar, although most species of Potiarca are less elongate. The cardinal area of both is wide and triangular. The progressively greater overlap of the left valve-progressively greater from the anterior end to the posterior end of the ventral margin-is the most characteristic shell feature of Cunearea, whereas the overlap of Potiarca is less pro-


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