OF SOUTHERN INDIA. 367 



ridge ; pallial impression simple or sinuated ; type, D. polymorpha. Pall. There 

 are said to be about eighteen recent species known from Africa and America, and 

 about twelve fossil, most of them occurring in the fresh and brackish water, upper 

 miocene, deposits of Central and Eastern Europe. 



H. and A. Adams propose for a number of recent species of the type of 

 -D. Africana, van Bened., the sub-generic name Praxis on account of the presence 

 of a certain small lamina affixed to the septum. I do not know what the real 

 character of this lamina is, (for I have none of those species to compare), but if 

 reference is made to the small projecting pit near the septum, which is so well 

 developed in all the tertiary fossil species described by Partsch, who specially 

 alluded to it in his description of the genus, the name Congeria has priority before 

 that of Praxis and should be reserved for that group of shells, for the pit does 

 not exist in the type species of recent Dreissena. fCongerla, vide Homes, Eoss. 

 Moll, des Wiener Beckons, vol. ii, p. 360). It appears to be this small pit attached 

 to the septum to w4iich Conrad in his newly proposed genus Ilytilopsis makes 

 reference. His characteristic (Proc. Phil. Acad., 1857, p. 167,) is as follows: 

 " Shell mytiliform, attached by a byssus ; hinge with a septum beneath, which on 

 the cardinal side is a triangular cup-shaped process ; cartilage groove rather deep ;" 

 type, Mijtilus leucophceatus, Con., from the rivers of Virginia. 



Thus the name Dreissena will stand for the type species B. polymorpha, 

 and Tichogonia is a synonym of it. Congeria will have to be retained ( ? sub- 

 generically) for the species of the type of Cong, sub-globosa, spathulata, &c., of 

 Partsch, and the names Praxis and Mytilopsis appear to be synonyms of this 

 second type. 



Euchs, in a recent communication* about some upper tertiary fossils from the 

 Banate, (Hungary,) notes a Dreissena Schrdclcingeri, in which the septum appears 

 to be transformed into a regular large anterior muscular scar, and the pallial line 

 shows a deep posterior sinus. StiU more recently! the same author proposes for 

 this species the new generic name Dreissenomya. 



b. Snb-familj/—CR.ENELLIN^. 



Shell elongately tumid, thin, with sub-terminal slightly swollen beaks, two 

 muscular scars, of which the posterior is larger, outer surface of valves entirely or 

 partially radiately striated (except in MyrinaJ ; hinge line often denticulate; 

 Ligament almost quite internal, in a linear groove, more or less extending 

 posteriorly. 



6. Crenella, Brown, 1827. Shell oval or rhombic, thin, moderately inflated, 

 radiately striated, hinge with one tooth in each valve, simple or crenvilated, and 

 produced parallel to the hinge margin ; ligament very thin, sub-internal, supported 

 by a distinct plate ; muscular scars two, faint, unequal ; pallial line entire, indis- 

 tinct; type, Cr. decussata, Montg., recent from British seas. Eossil species occur 

 in tertiary and cretaceous deposits, bvit those from older ones are doubtful. 



* Verhandlungen der geol. Eeichsanstalt, April, 1870, p. 97. t Ibidem, p. 320. 



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