350 THE OSTRACODERMS. 



no closed peribranchial chamber, the gills probably lying on the oral surface, 

 beneath the posterior part of the mesocephalon. (Fig. 232.) 



The trunk is short and slender, generally triangular in cross-section. It may 

 be practically naked, or provided with minute, scattered tubercles only (Bothrio- 

 lepis) ; or covered with rounded, overlapping scales (Pterichthys) ; or with large 

 segmentally arranged oblong plates on the neural surface, and small irregular 

 ones on the haemal side (Cephalaspis) ; or with shagreen-like denticles (ccelolepidae). 

 There are one, or two, unpaired dorsal fins, stiffened by delicate internal rays 

 (Bothriolepis), or by minute oblong dermal plates (Cephalaspis). Pectoral and 

 pelvic fins are absent. 



Lateral Fold. A narrow fold extends along the ventro-lateral margins of the 

 trunk. It may be entirely membranous (Bothriolepis); or supported by minute 

 rays (Pterichthys and one species of Cephalaspis) ; or it may be formed by the pro- 

 jecting ends of segmental trunk plates (Tremataspis) ; or it may consist of a series 

 of segmentally arranged, separately movable, appendage-like plates, or fringing 

 processes (Cephalaspis). 



The Cephalic Appendages. Large, oar-like cephalic appendages form one 

 of the most striking features of the ostracoderms. In Bothriolepis they are at- 

 tached to the posterior haemal margin of the mesocephalon, in front of the gills, 

 and consist of two joints, or segments, covered with bony plates. They are hollow, 

 triangular in cross-section, and contain indications of a cartilage axis. (Fig. 257.) 

 An opening on the posterior proximal end of the arm, and an adjacent one on the 

 side of the branchiocephalon, serve for the passage of nerves, blood-vessels, and 

 other tissues. 



In Cephalaspis the cephalic appendages are covered with minute, semi-isolated 

 dermal plates, and their broad distal ends, of undetermined contour, are horizon- 

 tally flattened. Parts of armored cephalic appendages similar to those of Bothrio- 

 lepis have been found in Palaeaspis, Cyathaspis, Tremataspis, and Psamosteus. In 

 Pteraspis, Drepanaspis, and Berkenia, there are certain marginal notches, or open- 

 ings, that have been considered as lateral eye orbits, but which may possibly 

 represent the points of attachment of cephalic appendages. In these genera they 

 were smaller, less heavily armored, if they were armored at all, and were not used 

 as swimming oars. 



The cephalic appendages of the ostracoderms are not comparable with the 

 pectoral fins of vertebrates, but with one of the pairs of thoracic swimming legs 

 of the merostomata. They are represented in vertebrates by the so-called " bal- 

 ancers," and the cephalic tentacles of amphibian larvae, and by similar processes 

 in certain fishes, i.e., Protopterus, Accipenser and Bdellostoma. 



Jaws. The mouth lies in a membranous portion of the haemal surface, cau- 

 dad to the projecting rostrum, or to the anterior margin of the procephalon. In 

 Bothriolepis, there are three pairs of bony plates, which represent, in part, the 

 premaxillary, maxillary, and mandibular arches of vertebrates. 



The maxillary arch is probably represented by the small movable plates (Figs. 



