xx OSTRACODERMI ANASPIDA 5 3 I 



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 Liposarcus), 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 

 improbable that the Ostracodermi are outcasts from the Elasmo- 

 branchs, a degenerate race which has sought safety in a sequestered 

 life and a coat of mail. 



Order III. Anaspida. 



This group has been instituted by Traquair l 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, us 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-like. 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. 



