SILURIAN FAUNAS. 285 
ACTINOPTERIA RETICULATA MN. Sp. 
5 ; Plate XXIL., Fig. 3. 
Description.—Left valve large, subrhomboidal in outline; the body 
subovate, with an obliquity of 27° between the hinge-line and the 
umbonal ridge; beak but slightly elevated above the hinge-line. An- 
terior margin nearly straight or slightly simuate, forming a rounded 
angle of about 80° with the hinge-line; basal margin regularly curved ; 
posteriorly it rounds more abruptly into the nearly straight or slightly 
sinuate posterior margin, which is nearly parallel with the anterior 
margin. Posterior wing large, shghtly convex, limited below by a 
moderately distinct, rounded sinus; the posterior cardinal extremity 
obtuse. The body of the shell is marked by conspicuous, concentric 
lines of growth and by more or jess discontinuous, radiating coste. 
Near the margin of the shell the concentric lines are more crowded 
_and the radiating coste are less conspicuous, but on the upper portion 
of the shell the two sets of markings give to the surface a nodose 
appearance. ‘The posterior wing is marked like the body of the shell, 
but the radiating markings are much less conspicuous. Right valve 
unknown. 
The dimensions of the type specimen are: height, 35 mm.; oblique 
jength from beak to postero-basal extremity, 48 mm.; length of hinge- 
line, 29 mm. 
Remarks.—Nothing but the general outline and surface markings 
of this shell are preserved, but in these characters 1t seems to resemble 
more closely the species of Actinopteria than any other generic group, 
and it is consequently placed in this genus, although no members of it 
have before been recognized earlier than the middle Devonian faunas. 
Additional material, more perfectly preserved, may serve to change 
its generic reference. 
MYTILARCA OBLIQUA N. sp. 
Plate XXII., Fig. 8. 
Description.—This species has been observed only in the form of 
somewhat indefinite casts, in which the hinge characters are not pre- 
served. The general form of the shell resembles Marctirostra H. and 
