Xx OSTRACODERMI—ANASPIDA 531 
other hand, its close resemblance to the Coelolepids in the general 
contour of its laterally-lobed body, and the probability that its 
mosaic and tuberculated head-shield has been formed by the 
concrescence of Coelolepid denticles, is at least significant of a 
relationship to the more primitive Heterostraci. Little can be 
conjectured as to the habits of these ancient “ Fishes.” The form 
and regional proportions of the body, which in some respects 
often remind one of organisms so diverse as a King Crab, or a 
Loricaroid Teleost (such as Ziposarcus), are strongly suggestive 
of a grovelling, bottom-feeding, sluggish habit of life, in sharp 
contrast to the more active and predaceous Fishes whose appear- 
ance is coincident with the extinction of the Ostracodermi at 
the close of the Devonian period. Habits such as these may 
well be associated with much structural degeneration, even, it 
may be, with the loss of paired fins, and hence it is not altogether 
unprobable that the Ostracodermi are outcasts from the Elasmo- 
branchs, a degenerate race which has sought safety im a sequestered 
life and a coat of mail. 
Order III. Anaspida. 
This group has been instituted by Traquair? for the pro- 
visional reception of two remarkable genera, which, owing to the 
absence of precise knowledge of the histology of their exo- 
skeletal structures,cannot at present be referred either to the 
Heterostraci or the Osteostraci, and for which, as their discoverer 
remarks, no place can be found in the system unless they are 
admitted to the Ostracodermi. 
Fam. 1. Birkeniidae.— Body fusiform and fish-lhke. Head 
bluntly rounded, without a cranial shield. Caudal fin bilobate 
and heterocercal. A median row of scales with recurved spines 
arranged along the ventral surface. Orbits, jaws, teeth paired 
fins, and endoskeleton unknown. 
In Birkenia (Fig. 320) the body is invested by longitudinal 
rows of narrow scales arranged in oblique transverse rows, which 
are replaced on the head by much smaller, peculiarly disposed, 
spindle-shaped scutes. On the side of the hinder part of the head 
there is an oblique row of small apertures, possibly branchial. A 
small remote dorsal fin, invested by the trunk scales, is present. 
1 Traquair, op. cit. p. 837. 
