THE ANTIARCHA. 367 



five- or six-sided plates. Peribranchial chamber opening on the posterior lateral 

 side of the head, between the posterior lateral and the posterior ventro-lateral 

 plates. Devonian. Drepanaspis, Schliiter. (Fig. 246.) 



IV. Antiarcha. 



Head oval, pentagonal in cross-section. Mesocephalon united with branchio- 

 cephalon by a transverse, movable joint. Cephalic armor consisting of large 

 plates; separate, but with little or no movement. Trunk slender, membranous 

 or scaly; triangular in cross-section, with distinct but narrow lateral folds. Two 

 dorsal fins, membranous, with or without supporting rays. Tail long, ending 

 in a narrow ribbon. Cancellae in one or many layers, often small and irregular. 

 Dermal armor ornamented with tubercles or, with concentric tuberculate ridges. 

 Dentine layer ill defined, thin, or absent. Lacunae highly developed, multipolar, 

 and extending close to outer surface. Large atrial and pre-oral chambers. 



The best known form is Bothriolepis canadensis, Whiteaves. The author 

 has secured a large number of these fossils, splendidly preserved. Large slabs 

 were obtained showing many entire individuals in their exact attitudes and 

 surroundings at the time of death. With this material at hand, which in abun- 

 dance and in perfection of preservation has never been equalled, it has been 

 possible to form a very accurate idea of the structure and mode of hfe of this 

 most interesting animal. 



The details of its anatomy have been worked out by means of serial sections 

 and by other methods. They will be reserved for a separate publication; we 

 have space here for only the points of general interest, or those bearing on the 

 subject under discussion. 



Exoskeleton. — The outer surface of the dermal armor was ornamented with 

 low rounded tubercles, arranged in concentric rows, often parallel to the margins 

 of the separate plates, or forming wavy, crenulate ridges. 



The dorsal mesocephalic shield moves up and down, to a limited extent, on 

 the hinge-like joint connecting it with the branchiocephalon. This movement 

 is made possible by the tilting of the suspensory or suborbital plate, whose thick, 

 ventral edge rests on the anterior lateral margin of the fixed, ventral shield (an- 

 terior ventro-laterals). (Figs. 253, s.o, 259, A.) When the mesocephalon is de- 

 pressed, the dorsal edge of this plate swings inward, without dislocating the plate, 

 through an angle of almost forty-five degrees. In partly crushed heads it is 

 forced into an abnormal, horizontal position, and lies inside the head with its 

 lateral surface turned dorsally. 



As the ventral edge of the suspensory plate fits into a shallow groove, or 

 ridge, its posterior end cannot normally swing bodily outward, after the fashion of 

 an operculum, as one might at first sight suppose. In fact, there is no passage- 



