On the Generic Allocation of “Aligena” Pisum Dall, 1908

additional records. Harry (1969), in the context of a systematic revision of the genus Aligena Lea, 1846, concluded that Aligena pisum was wrongly located under this genus, and proposed its tentative reallocation in the genus Axinulus Verrill & Bush, 1898, a decision not followed by cies under Aligena. Bernard (1983) considered Aligena pisum as a nomen dubium. The genus Adontorhina was proposed by Berry (1947) to include Lower Pleistocene specimens of his new species A. cyclia. The species was subsequently reported as also living by Howard (1952). Other species currently regarded under Adontorhina are: A. sphaericosa Scott, 1986, A. lynnae Valentich Scott (in Coan, Valentich Scott & Bernard), 2000, A. keegani Barry & McCormack, 2007, and A. similis Barry & McCormack, 2007. In this paper, Aligena pisum is redescribed, properly illustrated, and reallocated to Adontorhina. MALACOLOGIA, 2011, 53(2): 373 378


INTRODUCTION
Between 1887 and 1888, the United States Albatross conducted an expedition to the southern tip of South America. The bivalves from that expedition were studied by Dall (1889Dall ( , 1890Dall ( , 1896Dall ( , 1901Dall ( , 1902Dall ( , 1908Dall ( , and 1915, who reported 24 new species from the Magellanic region. However, the descriptions of many of them were poor and lacking in details and 17 This is the case, for instance, of Aligena pisum Dall, 1908, a species described based on a single left valve. Dall's description refers to a small, suborbicular shell, "striately" sculptured, hinge bearing "a single obscure tooth or nodular prominence". Although the species is mentioned in several subsequent checklists for the area additional records. Harry (1969), in the context of a systematic revision of the genus Aligena Lea, 1846, concluded that Aligena pisum was wrongly located under this genus, and proposed its tentative reallocation in the genus Axinulus Verrill & Bush, 1898, a decision not followed by cies under Aligena. Bernard (1983) considered Aligena pisum as a nomen dubium.
The genus Adontorhina was proposed by Berry (1947) to include Lower Pleistocene specimens of his new species A. cyclia. The species was subsequently reported as also living by Howard (1952). Other species currently regarded under Adontorhina are: A. sphaericosa Scott, 1986, A. lynnae Valentich Scott (in Coan, Valentich Scott & Bernard), 2000, A. keegani Barry & McCormack, 2007, and A. similis Barry & McCormack, 2007. In this paper, Aligena pisum is redescribed, properly illustrated, and reallocated to Adontorhina.

MATERIALS AND METHODS
The specimens reported in this paper were Chile, Beagle Channel, and Magellan Strait. Samples were obtained by using a dredge of ment by using a stereoscopic microscope. The number of specimens (s), articulated valves (av), and disarticulated valves (v) is provided. These specimens were deposited in the collections of the Museo de La Plata (MLP) and Museo Argentino de Ciencias Naturales "Bernardino Rivadavia" (MACN), Argentina. For comparative purposes, the holotype of Aligena pisum, housed at the United States National Museum (USNM), Smithsonian Institution, Shell morphology was studied and illustrated with Scanning Electron Microscopy (SEM). Shell measurements were taken according to the following criteria: L: maximum anterodistance across closed valves. The mean and the number of measurements (n), are given.
Information on gross anatomy comes from dissection under stereoscopic microscope of performed by rinsing the specimens in 5% formalin with 2% acetic acid.

Generic Placement
The hinge of Adontorhina pisum was previously misunderstood. Dall (1908) described it as having "a single obscure tooth or nodular prominence", and consequently placed the species under Aligena. Harry (1969) regarded such prominence as "a single irregular tooth … immediately below and in front of the umbo. The hind part of the tooth may be a broken, several rounded bosses, one of which is on the top side. A triangular extension forward of this tooth, has a moderately sharp ventral border, along which are several incised lines sloping upward and backward". Based on this interpretation, Harry (1969) tentatively reallocated the species to Axinulus. However, the hinge plate in Axinulus is completely smooth or bears a single "cardinal" tubercle (Scott, 1986). This is not the condition present in "Aligena" pisum, where several "granules" appear. These granules are stronger beneath the beak -corresponding to what Harry (1969) interpreted as "rounded bosses" -and anteriorly become slender, running parallel to the dorsal margin (i.e., the "triangular extension" and "incised lines" of Harry (1969)). This peculiar hinge plate morphology reveals that the species actually belongs to Adontorhina, the single thyasirid Barry & McCormack, 2007).

Comparison with Other Adontorhina Species
Adontorhina pisum closely resembles A. cyclia, from which it differs by having subquadrate and wide depressed area anterior to beak. In addition, the granules present in the anterior part of hinge plate of A. pisum are elongate and narrow, whereas those of A. cyclia are shorter and solid, resembling "mammalian molars", such as described by Scott (1986). Furthermore, the foot of A. pisum has a distinct bulbous tip which, according to Scott (1986), is absent in A. cyclia.
Adontorhina lynnae, A. keegani, and A. similis differ from A. pisum in their ovate shell outline, originated by their widely projected anterior end. According to Barry & McCormack (2007) the hinge plate of Adontorhina similis has an additional "small cardinal tubercle in the right valve", which is absent in A. (Zelaya, 2009(Zelaya, , 2010.
The authors express their gratitude to Fundación San Ignacio del Huinay, for help the Área Naval Austral, Tierra del Fuego, for the facilities to obtain samples at the Beagle ditional specimens from the Beagle Channel. Our gratitude also to Sankury Pye, for kindly sending the holotype of Aligena pisum, and Paul Valentich-Scott for valuable comments on the genus Axinulus.
This study was partially funded by PICT 282-2006. M. Güller is a fellowship student at Universidad de Buenos Aires, and D. Zelaya is member of the Consejo Nacional de Investi-LITERATURE CITED