Page:Journal of the Asiatic Society of Bengal Vol 7, Part 1.djvu/483

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1838.]
421
a genns of Fluviatile Testacea.


the beaks : on the other hand in the Nymphacece* r , and especially the true Donacidce, such as Donax, Capsa, &c. the ligament occupies the place of the lunule, and the siphons are exserted from the contraverse side or that towards which the beaks incline, In the fluviatile Concha the siphons are very short, and, as a necessary consequence, the siphonal scar is either very short or not apparent ; while in the Nymphacece it is very conspicuous, reaching in Capsa and Donax to the centre of the shell, and giving certain evidence of an elongated siphon. In these important particulars Galathea agrees with the Donacidce and differs from the Concha. Its ligament occupies the lunule instead of surmounting the corselet : the siphons occupy the contraverse side of the beaks ; and lastly the elongated siphonal scar, indicative of a strong distinctive character in the animal, decides its location apart from the < group which includes Cyrena. Its analogies also have reference to the Donacidce in the peculiar truncation of the lunular side for which that family is remarkable.

An examination of the teeth of Galathea, will further shew its relation to the Donacidce and its separation from Cyrena . In Cyrena the three primary teeth are alike divergent, seeming like remote elided portions of rays proceeding from a common centre. In Galathea and Capsa the primary teeth in the left valve consist of two radiating prominences nearly joined at their points of departure, and exhibiting, in the included hollow space, an obtuse triangular tooth ; in the right valve there are two similar but more closely approximated primary teeth, with an interjacent hollow fitted to receive the triquetrous tooth of the opposite valve.

In Capsa and Galathea the ligament is singularly short, occupying a broad space close to the beaks, and immediately over the cardinal teeth. In Cyrena this important apparatus is removed towards the posterior lateral tooth.

Lamarck following Bruguieres, decided on the separation of Gala- thea from Cyrena, from the consideration of the form and position of the teeth alone. The particulars now stated, will shew how well those characters might have been relied on even for a more distant separation.

  • NoTE.—Astarte of Sowerby (Crassina of Lamarck) is out of its place among the Nymphacece. Its hinge and siphons both refer to the Concha.

f Note.— In this comparison I have the typical species of Cyrena in view, such as Cyrena Sumatrana, so commonly imported into Calcutta from the Sundurbuns, for the purpose of reduction into lime, and not the aberrant species with elongate serrated lateral teeth, which are so numerous in the upper portious of the Gangetic branches, and which constitute the genus Corbicula of Megerle