SUB-CLASS II 



OSTRACODERMI 



57 



covered with four pairs of lateral plates and four unpaired median plates, 

 which are united by straight sutures. On the lower surface the cleft for the 

 mouth is bounded in front by a pair of transversely elongated plates, which 

 are sometimes supposed to represent the upper jaw. Behind the cleft a pair 

 of smaller plates occurs fixed to the front of the abdominal shield. The 

 trunk is armoured above by two 

 large median and two pairs of 

 lateral plates ; on its ventral surface 

 four large ventro-laterals Surround 

 a small rhombic ventro - median 

 element. The ventro-laterals are 

 sharply bent upwards and outwards 

 near their lateral border to take 

 part in covering the flank. Both 

 the head plates and the dorso-lateral 

 plates of the trunk are traversed by 

 sensory grooves, those of the head 

 being united by two parallel com- 

 missures. The two hinder lateral 

 plates of the head and the adjoin- 

 ing body plates have bevelled edges, 

 so that the former overlap the latter. 

 The anterior median dorsal plate of 

 the trunk somewhat overlaps the 

 lateral plates, and is overlapped 

 behind by the border both of the 

 posterior dorso-laterals and of the 

 posterior median dorsal plate. The 

 orbits occur at the two ends of a 

 transverse cleft on the upper sur- 

 face of the head, and are separated 

 by a quadrangular median plate (os 

 duhium or os pineale), which exhibits 

 a small deep pit on its inner face. 

 The two anterior ventro - lateral 



Fig. 100 



Pterichthys miUcri, Ag. Lower Cid Red Sandstone ; 

 Gamrie, Scotland. A, Specimen in nodule, 1/2 nat. size. 

 B, Scale enlarged (after Egerton). 



plates of the trunk are provided in front with an articular hollow on each 

 side, in which are fixed the armoured paddle-shaped pectoral appendages. The 

 latter do not reach the hinder end of the body shield, and exhibit one trans- 

 verse articulation (j) at about their middle. The tail is covered with thin, 

 rounded, or six-sided, overlapping scales. The single dorsal fin is small. 

 The tail is heterocercal, with large fulcral scales on its upper border, and the 

 fin membrane confined to the lower lobe. P. milleri, Ag., and other species 

 represented by complete but usually ill-preserved examples in the Lower Old 

 Red Sandstone of Scotland. Fragments in the Middle Devonian of the Eifel. 

 Asterolepis, Eichw. (Narcodes, Odontacanthus, Ag.). As Pterichthys, but often 

 larger, and the anterior median dorsal plate overlaps both the anterior dorso- 

 laterals and the posterior dorso-lateral plates. Chiefly represented by detached 

 plates in the Devonian of N.-W. Russia and in the Upper Old Red Sandstone 

 of Scotland. More doubtful fragments from the Devonian of Bohemia, the 

 Eifel, East Greenland, and Australia. 



