LAMELLIBRANCHIATA. 609 
Lyrodesma acuminatum.] 
Beaks small, placed in front of the midlength; posterior umbonal ridge generally 
prominent, often angular; post-cardinal slope frequently with radiating lines, the 
rest of the surface with concentric striz only. Hinge consisting of from six to eight 
prominent, subequal, transversely striated teeth, radiating regularly from the beak 
and placed on a thick plate, which leaves a large oblong depression in the dorsal 
edge of casts of the interior. Adductor scars rather faintly impressed, the posterior 
one larger than the anterior. Two pairs of small pedal muscles, the anterior pair 
situated immediately above the anterior adductors, the posterior pair on eath 
side of the hinge line just behind the hinge teeth. Pallial line shghtly sinuate 
posteriorly. 
Type: L. planum Conrad. 
Of this excellently marked genus I know eleven or twelve American specific 
forms. Hight of these occur in the various horizons of the Cincinnati group, the 
remainder in the Trenton. Two additional species are catalogued by Bigsby among 
the European Lower Silurian shells. : | 
LyRoDESMA ACUMINATUM, %. Sp. 
PLATE XLII, FIGS. 1—. 
Shell obliquely acuminate-ovate, the outline being drawn out to an acuminate 
extremity posteriorly; in the typical form (fig. 1), the hinge line is arcuate and passes 
gradually into the posterior margin, which, because of the flattening of this region 
projects, in a side view, but little beyond the sharply angular umbonal ridge; anterior 
end broad, regularly rounded; base straight posteriorly. Beaks small, arcuate, 
strongly incurved, not very prominent, situated somewhat less than one-third of the 
length from the anterior extremity. Surface with obscure, distant, concentric lines; 
on the posterior cardinal slope four or five radiating lines. Hinge with six teeth of 
which the anterior ones are considerably shorter than the posterior one, and the 
central ones curved backward. Posterior adductor impression unusually distinct; 
sinus in pallial line very small. 
The specimen represented by figures 3 and 4 (plate XLII), is one of several that 
I refer to this species with considerable doubt. The posterior end is too short caus- 
ing the beaks to be more central, and the post-cardinal margin is more prominent 
and subalated. The umbonal ridge is even sharper and more prominent, its greater 
distinctness being due to a somewhat greater flattening of the flanks of the valves. 
The hinge is injured in the specimen, but it is quite evident that the teeth have not 
that backward sweep which marks the typical form. Precisely the same form (see 
figure 45-h, page 611) occurs in the Trenton of Kentucky, but, so faras known, it is 
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