262 



ZOOLOGY 



SECT. 



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

 genera are known of Silurian and Devonian age. 



The Drepanaspidce (Fig. 918) have a somewhat similar shape, but with the 

 head and trunk expanded into a broad shield, which is sharply marked off from 

 the tail. The exoskeleton consists of scales and fulcra (see p. 228), replaced 

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

 sides by postero-lateral plates (p. I. ). 

 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 



FIG. 917. Restored outline of Zianarkia 

 spinosa, in the position in which it 

 occurs as a fossil, the head being flat- 

 tened and the tail twisted round so as 

 to appear in profile. On each side a 

 much enlarged dermal denticle is shown. 

 (From the Cambridge Natural History, 

 after Traquair.) 



FIG. 918. Restored outline of the dorsal sur- 

 face of Drepanaspis gemundenensis. 



The tail appears in profile, m. d. median 

 dorsal plate ; p. I. postero-lateral plate ; 

 r. rostral plates. (From the Cambridge 

 Natural History, after Traquair.) 



of certain fragmentary remains in the form of dermal plates which closely 

 resemble those of the Drepanaspidae. 



ORDER 2. OSTEOSTRACI. 



Cephalaspis (Fig. 919) 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 



