532 FISHES CHAP. 
Birkenia elegans, the only species known, does not exceed 35 inches 
in length. Less is known about the second genus, Lasanius, of 
which there are two species. Except for the mid-ventral series 
of spiny scutes, and a row of slender, parallel, rod-like structures 
Fic. 320.—Restored outline of Birkenia elegans Traq., one-half larger than natural size. 
b.a, Branchial aperture ; d, dorsal fin. (From Traquair. ) 
the body appears to have been naked (Fig. 321). The two genera 
belong to the remarkable series of fossil Fishes from the Silurian 
rocks of Lanarkshire. Rare in the Ludlow series, Birkenia is by 
far the most common of the Fishes of the over-lying Downtonian 
Fig. 321.—Restored outline of Lasanius problematicus, enlarged. 7, Post-cephalic rods ; 
vr’, row of small spine-like scutes; v.s, mid-ventral spine-like scales. (From 
Traquair. ) 
Beds. Zasanius is confined to the latter horizon. Huphanerops, 
from the Upper Devonian of Canada, is probably related to this 
family, but lateral branchial apertures are not known.! 
III. Antiarchi. 
The organisms comprising this group” resemble the Ostra- 
codermi in possessing a well-developed carapace of bony plates 
and a heterocercal tail, as well as in many of the purely 
_hegative features which are characteristic of the latter group. 
The remarkable dorsal shield is divided into a small cephalic 
4 Smith Woodward, Ann. Nat. Hist. (7), v. 1900, p. 416. 
2 Traquair, Monogr. Palaeont. Soc. 1894. 
