^ , LAMELLIBEANCHIATA. 609 



ijyroaesma 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 striae only. Hinge consisting of from six to eight 

 pl-ominent, 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 each 

 side of the hinge line just behind the hinge teeth. Pallial line slightly sinuate 

 posteriorly. 



Type: L. planum Conrad. 



Of this excellently marked genus I know eleven or twelve American specific 

 forms. Eight 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. 



Ltrodbsma acuminatum, n. sp. 



1 PLATE XLII, FIGS. 1—5. 



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-A, page 611) occurs in the Trenton of Kentucky, but, so far as known, it is 



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