DREISSENA. 477 
the common Mussel (Mytilus) ; inside, a peculiar wall (septum) in the umbonal region. 
vo adductors, the anterior very small; no pallial sinus. Borders of the mantle closed 
in half the length of the underside, leaving an opening for the cylindrical foot, which 
spins a byssus, like the common Mussel. Ventral margin of the shell somewhat 
asymmetrical. 
Found in various parts of the world, chiefly in tropical and subtropical regions ; one 
species widely spread in Europe by human intercourse, by vessels and rafts, since 1824. 
Tretain the usual spelling, Dreissena, although the individual after whom it was named 
called himself ‘Dreissens”; but for the sake of euphony it is perhaps permissible to 
alter the spelling somewhat, as in Rissoa, Desmoulins, for Rissoia, Draparnaldia, Bory 
St. Vinc., for Draparnaudia, &c. 
Subgen. PRaxis. 
Praxis, H. & A, Adams, Gen. Moll. ii. p. 522 (1858). 
A spoon-like appendage at the upper end of the septum. 
Chiefly American. 
1. Dreissena sallei. (Tab. XXXII. figg. 4, 5.) 
Dreissena sallei, Récluz, Rev. Zool. 1849, p. 69'; Journ. de Conch. iii. p. 258, t. 10. fig. 9 (1852) *; 
Dunker, De Septiferis et Dreissensiis, p. 18 (1835) °. 
Dreissensia sallet, Fisch. & Crosse, Miss. Scient. Mex., Moll. ii. p. 504, t. 62. fige. 4, 4a, 6, 5*. 
Mytilus sallei, Reeve, Conch. Icon. x., Mytilus, tig. 44 (1858) °. 
Tichogonia sallei, Clessin, in Martini & Chemnitz, Syst. Conch.-Cab. ed. 2, Mytiliden, p. 17, t. 12. 
figg. 18, 14 (1889) °. 
Moderately convex, somewhat roughly striated, dull, pale greyish-brown, either unicolorous, or with darker 
shades parallel to the lines of growth, or with scattered darker brown spots arranged along the lines of 
growth; a distinct ridge separating the sides from the base only near the summits, evanescent in the 
greater part of the shell; summits bent down ; fore half of the dorsal margin more or less ascending in a 
slightly convex line, hinder half less steeply descending, hinder extremity rounded; ventral margin 
concave only in its fore half. Angle between the fore and hinder parts of the dorsal margin in most 
specimens very obtuse, rarely more distinct. Inside pale violet, with darker blotches, especially along the 
pallial line; septum roundly notched, with a somewhat triangular (rather than spoon-shaped) appendage, 
scarcely excavated, directed towards the lateral wall of the shell. 
a. Long. 19-23, alt. 13-14, diam. ? millim. 
be 4, 23, » 134, eae! ree 
C. ” 22, ” 10, ” 9 ” 
d. 9 26, ” 13, 2 9 9 (Fig. 5.) 
é. ” 19, ” 12, ” ? ” 
ft 29 20, ” 123, ” 74 ” (Fig. 4.) 
a. Measurement given by Récluz*; 6. Fischer & Crosse’s fig. 4; c. Specimen from Guatemala in Dunker’s 
collection; d and f. Specimens from Honduras; ¢. Fischer & Crosse’s fig. 5; ¢ and f are more elevated specimens, 
with distinct dorsal angle. 
Hab. E. Guaremata: Rio Dulce and the Lake of Yzabal, in freshwater, attached to 
rocks and to one another (Sadlé*~*). 
Honpvras (coll. Dunker). 
