PTERASPIDOMORPHI 



197 



Family DREPANASPIDAE. The expanded depressed body is shaped 

 much as in Thelodus ; but the tail, although heterocercal, is not so 

 distinctly bilobed, and no dorsal fin has been found (Fig. 169). The 

 whole animal is covered with plates and scales, ornamented with stellate 



FIG. 167. 



Diagrammatic restoration of Lanarkia spinosa, Traq. Dorsal view with the tail twisted to 

 show the heterocercal caudal fin. The eyes appear near the anterior border. (After Traquair, 

 Trans. Roy. Soc. Edin.) 



tubercles or ridges, A large median dorsal plate is situated" above, two 

 ' rostral ' plates at the anterior edge ; a large median ventral plate occurs 

 below, with a median ' mental ' in front. The mouth was situated in 

 the transverse slit between the rostrals and the mental ; and a cloacal 

 aperture seems to have been placed behind the ventral. Anterior and 



Fio. 168. 



Diagrammatic restoration of Thelodus scoticus, Traq. Dorsal view with the tail twisted to 

 show the caudal and dorsal fins. The eyes are shown near the anterior border. (After Traquair, 

 Trans. Roy. Soc. Edin.) 



posterior ventro-lateral plates occur, also a pair of lateral plates are 

 pierced by a hole, which probably represents the orbit. The eyes were, 

 then, rather ventral than dorsal, and far apart. The lateral flaps, the 

 expanded edge of the body, are enclosed in long * postero-lateral ' plates. 

 The spaces between the large plates are covered with a mosaic of smaller 

 polygonal plates, which are continued on to the tail and caudal fin, where 

 they acquire the shape of imbricating scales diminishing in size behind. 



