OSTRACODERMI — HETEROSTRACI 



525 



from the Ludlow and Downtonian Beds of Lanarkshire. De- 

 tached scales are also known in the Upper Silurian of England. 

 One species {Th. pagei) occurs in the Lower Old Eed Sandstone 

 of Forfarshire, and another (Th. tulensis) in the Upper Devonian 

 of Kussia. Lanarkia has only been found in the Downtonian 

 Beds. None of the Coelo- 

 lepidae exceed fourteen to 

 fifteen inches in length. 



Fam. 2. Drepanaspidae. 

 — This family ^ affords an in- 

 teresting transition to the 

 more highly specialised and 

 carapaced Pteraspidae. The 

 head and anterior part of the 

 trunk now form a broad ob- 

 lons shield, rounded in front 

 and abruptly marked off from 

 the tail by conspicuous rounded 

 angles. The exoskeleton is no 

 longer uniform. In the caudal 

 region the scattered spines or 

 shagreen tubercles of the 

 Coelolepids have become trans- 

 formed into tuberculated quad- 

 rangular scales, which are 

 further differentiated along 

 'the dorsal and ventral margins 

 into ridge scales or fulcra ; 

 and from a similar source by a 

 process of basal fusion a series 

 of larger or smaller dermal 

 plates are formed as com- 

 ponents of large dorsal or 

 ventral shields. The dorsal 



shield (Fig. 314) is formed by a large central plate;' the 

 postero-lateral portions by two narrow falciform plates ; and 

 the anterior margin by a series of smaller rostral plates. 

 Between the larger plates the shield is completed by numerous 



' Id. Trans. Roy. Soc. Edinh. xxxix. 1S99, p. 844 ; Geol. Mag. vii. 1900, p. 153 y 

 ix. 1902, p. 289 ; Trans. Boy. Soc. Edinh. -xl. Pt. iv. 1903, p. 723. 



Fig. 314. — Kestoretl outline of the dorsal sur- 

 face of jyrepanaspis gemmuleuensis. The 

 tail appears in profile, m.d. Median dorsal 

 plate ; p.l, postero-lateral plate ; r, rostral 

 (From Traquair.) 



