JAWS. SKELETON. 351 



254, 259, mx.) The broad premaxil las, p.mx., have heavy crushing margins on their 

 median ends, and move laterally to and from the mouth, which lies in the median 

 line between them. The mandibles (Fig. 254, md.), are narrow, curved, and 

 pointed at their median ends, and form a thick bony covering to a hollow axial por- 

 tion that probably consisted of cartilages. They move diagonally forward and in- 

 ward, and backward and outward, pushing or scooping the food forward between 

 the premaxillae and toward the mouth, m. Behind the mandibles, om., the cir- 

 cumoral membrane is stiffened by two thin bands of bone which probably rep- 

 resent the dermal armor of the hyoid arches. 



In Tremataspis (Fig. 237), the oral region is occupied by flat polygonal plates, 

 showing little resemblance to jaws. They are arranged in four transverse rows, 

 which possibly represent the premaxillary, maxillary, mandibular, and two rows 

 of hyoid plates seen in Bothriolepis. 



In Cephalaspis there are indications of one pair of large crushing jaws. The 

 jaws of other members of the ostracoderms probably resemble those of Bothrio- 

 lepis or Tremataspis, but no traces of them have as yet been found. 



The Skeleton. The ostracoderms were no doubt provided with cranial 

 cartilages, including an endocranium and a notochord, but they were not volumi- 

 nous or resistant, for in Bothriolepis no certain traces of them can be seen, although 

 the skin and other soft parts are clearly indicated. In sections of the branchio- 

 cephalon small black rings are sometimes seen, mingled with the blackened rem- 

 nants of the gills and viscera, that may be fragments of the notochord sheath. 



The chief interest lies in the bony exoskeleton, which presents a structure in- 

 termediate between the chitenous epidermal skeleton of arthropods and the der- 

 mal skeleton of vertebrates. 



In a primitive ostracoderm, the general character of the external armor is 

 similar to that of a trilobite, or a merostome, in that it may consist of an almost 

 continuous shell, or buckler, for the broad cephalic and branchial regions, and seg- 

 mentally arranged plates, corresponding with the pleural and tergal plates, on 

 the flanks and dorsal surface of the trunk. The dorsal fin, all but the terminal 

 part of the tail, and the ventral surface of the trunk, may be covered with 

 minute oblong plates similar in structure and surface ornament to the larger ones. 

 (Cephalaspis.) 



In its simplest condition, the matrix of the dermal skeleton consisted of paral- 

 lel, or concentric lamellae, that apparently have been formed in the same manner as 

 the characteristic lamellas in chitenous exoskeletons. It is unlike chiten chemically, 

 but resembles it in the varying degree of hardness, color, and other optical proper- 

 ties of the lamella?, and in the presence of innumerable, parallel, unbranched 

 canals (pore canals, or primitive dentinal tubules) which everywhere penetrate the 

 matrix at right angles to the lamellas. 



The exoskeleton usually consists of three principal layers consisting of a bony 

 or dentine-like substance: a. an inner one of horizontal lamellas; b. a middle one 

 of large polygonal spaces, or cancellas, and c. an outer layer consisting mainly of 



