1838.] a genns of Fluviatile Testacea. 421 



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 

 3 g 2 



