532 



FISHES 



Birkenia elegans, the only species known, does not exceed 3^- 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 



FIG. 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. Eare in the Ludlow series, Birkenia is by 

 far the most common of the Fishes of the over-lying Downtonian 



FIG. 321. Restored outline of Lasantus prublematicus, enlarged. r, Post-cephalic rods ; 

 r', row of small spine -like scutes; v.s, mid -ventral spine -like scales. (From 

 Traquair.) 



Beds. Lasanius is confined to the latter horizon. Euphanerops, 

 from the Upper Devonian of Canada, is probably related to this 

 family, but lateral branchial apertures are not known. 1 



III. Antiarchi. 



The organisms comprising this group 2 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 

 negative . features which are characteristic of the latter group. 

 The remarkable dorsal shield is divided into a small cephalic 



1 Smith Woodward, Ann. Nat. Hist. (7), v. 1900, p. 416. 

 2 Traquair, Monogr. Palaeont. Soc. 1894. 



