PANAMIC-PACIFIC PELECYPODA 327 



Genus MACTRELLONA Marks, 1951 

 (Mactrella of most authors but not strictly of Gray, 1853.) 



Type species by original designation, Mactra data Spengler. 



Shell trigonal or hatchet-shaped, thin, inflated, with full prominent and 

 nearly median umbones and small, adjacent prosogyrate beaks. The surface 

 is smooth or marked with fine growth lines only. The posterior-dorsal area 

 is wide, flattened or slightly arched, marked off sharply by an angle or a 

 keel which may be elevated and winged. Tensilium attached to a short, 

 shelly plate forming a small roof over the chondrophore and formed as 

 an extension of the anterior margin back under the beak. Hinge concen- 

 trated, the posterior lateral tooth small, and crowded close against the side 

 of the chondrophore. 



The species of this group have generally been referred to Mactrella 

 Gray, 1853, for many years, including by Gray himself, in the general 

 belief that Mactra striatula, its type species by monotypy, was identical 

 or closely allied to Mactra alata Spengler. Gray's original description of 

 Mactrella seems certainly to refer to a species of the alata group (a thin, 

 cordate, triangular shell with a short, hinder, lateral tooth) but not to 

 M. striatula, the species he designated as type; it is clearly a case of species 

 misidentification. The M. striatula of Linnaeus, as this shell was figured by 

 Hanley, evidently from the type specimen, has an elongated form and 

 the umbones are waved, and it belongs to the group commonly known as 

 Mactrinula. Marks has recently discussed this subject (1951, Bull. Amer. 

 Paleont., vol. 33, No. 139, p. 355). If we must adhere strictly to the rule 

 that a designated species must remain the type of a genus, irrespective 

 of the author's description and real intentions, then Mactrella must fall and 

 become a synonym of Mactrinula. As a replacement, Marks proposed the 

 name "Mactrellona" which is used in this work under protest. 



Key to Panamic-Pacific species of Mactrellona 



I. Posterior-dorsal area wide and flat, bordered by a high, winged keel. 



1. Shell elongate-ovate, the two ends nearly alike, the basal margin 

 rounded without a marked bulge. No posterior gap. 



M. alata 



2. Shell higher, hatchet-shape, the ventral margin with a large bulge on 

 the posterior side of the middle. Posterior margin with a small open gap. 



M. clisea 



II. Posterior-dorsal area flat or slightly arched, bordered externally by a 

 small angle or ridge only, never alate. 



M. exoleta 



Mactrellona alata (Spengler) Plate 56, figure 4 



Mactra alata Spengler, 1802, Skrivt. Nat. Selsk., vol. 5, pt. 2, p. 99.— Reeve, 1854, 



Conch. Icon., vol. 8, Mactra, pi. 8, fig 29. — McLean, 1951, New York Acad. 



Sciences, vol. 17, pt. 1, p. Ill, pi. 23, fig. 1. 

 Mactra {Mactrella) alata Spengler, Dall, 189+, Nautilus, vol. 8, No. 3, p. 26. 

 Mactra {Mactrella) subulata Morch, 1861, Malak. Blatter, bd. 7, p. 180 Realejo, near 



Corinto, Nicaragua. 

 Mactra {Mactrella) alata Spengler, var. subalata Morch, Olsson, 1935, Nautilus, vol. 



48, No. 3, p. 105. 



