962 



CLASS PISCES. 



ever, unprovided with cornua ; while other genera are Thyestes and 

 Tremataspis, from the Silurian of Russia. 



Suborder 2. Placodermata. — This suborder is mainly char- 

 acteristic of the Devonian, although represented also in the Silurian, 

 and lingering on till the Permian. It is characterised by the carti- 

 laginous vertebral column, and the en- 

 closure of the head and the anterior part 

 of the body in bony plates, which are 

 covered with a radiate or granular sculp- 

 ture ; the tail being either naked or 

 clothed with scales. There is a distinct 

 mandible ; teeth are frequently present ; 

 and there may be a jointed pectoral fin 

 enclosed in a bony covering like that of 

 the body, but pelvic fins are invariably 

 absent. 



Family Asterolepidid^e. — In the 

 typical family the head (figs. 891, 892) 

 is rounded anteriorly, and covered with a 

 number of small thin plates ; while the 

 body is sub-quadrangular and invested 

 with larger plates, some of which are 

 median and others paired ; and the tail 

 may either be covered with much smaller 

 scales or naked. There is a well-devel- 

 oped pectoral limb, which articulates with 

 the anterior ventro-lateral plate ; although 

 it was long thought to articulate in some 

 forms to a separate thoracic plate. The type genus Asterolefiis, 

 which occurs both in Russia and Scotland, is characterised, accord- 

 ing to Dr Traquair, by the anterior median dorsal plate overlapping 

 both the anterior and the posterior dorsolaterals, and by the some- 

 what depressed body. In Pterichthys (figs. 892, 893), on the other 

 hand, the median dorsal plate, while overlapping the anterior dor- 

 so-laterals, is itself overlapped by the posterior dorsolaterals, and 

 the body is much more elevated. This genus has been recorded 

 from Scotland and the Eifel, and probably also occurs in Russia. 

 Detached pectoral fins of these genera have been described 

 under several names, and were at one time regarded as ichthyo- 

 dorulites. 



The genus Bothriolepis, from the Devonian of Europe and 

 Canada, is distinguished by the different contour of the cephalic 

 plates, and of the grooves of the lateral line system by which they 

 are marked, as well as by the shorter limbs. No traces of the scaly 

 tail have been observed in any of the known specimens, although 



Fig. 891. — Reduced restoration 

 of the dorsal aspect of Asterolepis 

 ornata', from the Devonian of 

 Russia. The tail is restored from 

 Pterichthys. (After Pander.) 



