366 CEETACEOUS PELECYPODA 



the ligamental furrow. The type species S. crassus, Sandb., is from Devonian 

 beds of Villmar. The genus differs from Dreissena merely by having the umbonal 

 plate attached chiefly along the dorsal side of the shell. 



2. (Anthracoptera, Salter, 1863). In connection with the last genus should 

 be mentioned the peculiar fossil described by Gabb apparently from the coal- 

 measures of North America as Myalina deltoidea, (Proc. Phil. Acad., 1859, p. 297, 

 and 1860, p. 55, pi. 7, fig. 1). The shell resembles the type of Hoplomytilus in 

 form, but the description of the hinge is not quite intelligible. Gabb says, "inside, 

 the cardinal third of the shell shows an alation which is invisible exteriorly." The 

 figure appears to indicate that a kind of lamina, or a thickening, extends from the 

 beak some distance along the dorsal edge, gradually becoming wider and passing into 

 the thickness of the shell. Salter figured a similar shell from the coal-measures 

 of Nova Scotia (Quart. Journ. Geol. Soc, Lond., xix, 1863, p. 79, fig. 3, and 

 p. 80), and suggested the name Anthracoptera for it, but of the only species 

 A. Carbonaria the hinge is, I believe, as yet unknown. It is possible that the two 

 species belong to a genus distinct both from the former and from the next. 



3. Myalina, Koninck, 1842, (An. foss. Carb. de Belg., p. 125). Elongately 

 sub-trigonal, with pointed beaks; each with a small internal septum, ligamental 

 area broad, with the ligament internal, and several longitudinal furrows between 

 it and the outer posterior margin; type, M, G^6>/<i/z^55m^(^, K., from carboniferous 

 rocks. The genus may be also represented in triassic rocks through such species 

 as Mytilus Milnsteri, Klipst., (see Laube in Denksch. Acad., Wien, xxv, pt. ii, 

 p. 44, pi. 16, fig. 5). It is said to belong to the Aviculid^, but the relation 

 which the typical forms show with species like Dreissena [Congeria] spathulata, 

 Partsch, clearly indicate the natural grouping of the genus. 



4. Septifer, Eecluz, 1848. Shell elongated, with the beaks pointed, anterior, 

 internally often with a few terminal small hinge-teeth and a large septum, 

 anterior ventral side with a byssal opening and usually incurved margins ; surface 

 mostly radiate] y striated and covered with a thin epidermis ; inner margin of shell 

 often denticulated; anterior muscular impression small, rounded, posterior elon- 

 gated and often somewhat irregular ; pallial line entire ; type, S. hilocularis, Linn. 



5. Dreissena,^ Van Beneden, 1835. Triangular or irregularly quadrangular, 

 with pointed beaks and a ridge running from it along the dorsal margin ; surface 

 smooth, with more or less distinct striae of growth, gaping at the anterior ventral 

 side; hinge usually with an apical tooth in the right valve; umbonal plate 

 occupying the whole of the antero-terminal part, more or less developed, and 

 posteriorly sometimes provided with a small pit, which is produced either by the 

 labial suspensor or the pedal muscle ; posterior muscular impression larger, rounded, 

 or oval ; ligament internal or sub-internal, internally accompanied by a longitudinal 



* Setting aside the right to priority of Mlinster's name EnocepJialus, the other three names, Dreissena, Tichogonia, 

 and Congeria, may be considered as possessing equal claim to be accepted, all having been published and equally well 

 characterized within the same year, 1835. From Fischer's historical account (Journ. de Conch., 1858, VII, 

 p. 125), it, however, appears that Van Bene den's name was published earliest. 



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