xm 



PHYLUM CHORDATA 



255 



externally with a layer of vaso-dentine. The tail appears to have been 

 heterocercal. A pair of longitudinal" ridges may represent paired fins. 



The Codolepidce (Fig. 928) have the head and anterior trunk region flattened 

 and expanded, with postero-lateral lobes which may represent paired fins. 

 There is a heterocercal tail-fin. Mouth, orbits, and branchial apertures have 

 not been detected. The exoskeleton takes the form of numerous uniform, 

 hollow, pointed spines, or tubercles, composed of dentine coated with ganoin. 

 Two genera are known of Silurian and Devonian age. 



The Drepanaspidce (Fig. 929) have a somewhat similar shape, but with the 

 he?,d and trunk expanded into a broad shield, which is sharplj' marked off 

 from the tail. The exoskeleton consists of scales and fulcra (see p. 222), 

 replaced in the middle of the dorsal surface by a large dorsal plate (m. d.) and 



op 



FIG. 930. A, restoration of shield of Cephalaspis lyelli, dorsal aspect"; ^/-diagram of ventra 

 aspect of shield of Cephalaspis; C, restoration of Cephalaspis murchisoni 

 (Devonian), op. opercular plate or pectoral fin ; or. orbit ; s. f. p. sub-frontal plate. (From 

 the Brit. Mus. Cat. of Fossil Fishes.) 



at the sides by postero-lateral plates (p. L). A similar combination of large 

 plates and small scales occurs on the ventral surface. The sole known 

 representative of the family is of Lower Devonian age. 



The family Psammosteidce has been formed for the reception of certain 

 fragmentary remains in the form of dermal plates which closely resemble 

 those of the Drepanaspidae. 



ORDER 2. OSTEOSTRACI. 



Cephalaspis (Fig. 930) may be taken as an example of the five genera 

 included in this order. The head is covered with a calcified shield, which has 

 a curious resemblance to the cephalic shield of Limulus or of a Trilobite, being 

 gently curved above, produced behind into spines, continued ventrally into a 

 sub-frontal plate (B, s. /. p.), and having a pair of orbits (A, or) for the eyes 



