262 



ZOOLOGY 



SKCT. 



auom. 



Two 



j)oiiitecl spines, or tubercles, composed of dentine coated Mitli 

 genera are known of Silurian and Devonian age. 



The Drepanaspida (Fig. 918) have a somewhat similar shape, but witli the 

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

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

 in the middle of the dorsal surface by a large dorsal ])lale (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 familj- PsammoMeidm has 

 been formed for the reception 



Fig. 917. — Restored outline of Iianarkia 

 spinosa, in the po.sition 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 CayabrUI'je JWttitrul JJistori/, 

 after Traquair.) 



Fig. 018. — Restored outline ol the dorsal sur- 

 face of Drepanaspis gemundenensis. 



The tail appears in profile, m. d. median 

 dorsal plate ; j'. I. postero-lateral jjlatc ; 

 r. rostral plates. (From the Cambi-idge 

 natural histori/, after Traquair.) 



of certain fiagmcntary icmains in the form of dermal plates which closely 

 resemble those of the Drupanaspid;e. 



Order 2. — Ostkostract. 



Ctpliala><pi>i (Fig. 919) may be taken as an example of the five genera included 

 in this order. Tlie head is covered with a calcified shield, which has a curious 



