200 



P TERA SPIDOMORPHI 



very large convex dorsal plate bears the spinal plate behind, and is flanked 

 on either side by a cornual or postero-lateral plate. The latter doubtless 

 represents the lateral flap in the Coelolepidae and 

 Drepanaspidae. A median internal depression be- 

 tween the orbits is taken to mark the position of 

 the pineal body ; and an oblique slit in each cornual 

 may represent a gill-opening. 



Fitting below the dorsal shield is a large convex 

 ventral shield formed of a single piece (Fig. 170, C). 

 The mouth was between it and the rostral. The tail- 

 region is covered with imbricating scales. An elabo- 

 rate system of lateral-line canals, opening to the 

 exterior by a double series of pores, is present in the 

 dorsal shield (Fig. 170, D, E) (A. S. Woodward 

 [503]). 



In microscopic structure the plates resemble those 

 of Psammosteus (p. 195); but the superficial placoid 

 scales have become converted into long ridges. The 

 caudal scales are of the same nature. 



In Cyathaspis the rostral shield is quite short, 

 and the orbits do not appear to have been enclosed in 

 from visceral surface of the carapace (Fig. 172). 



dorsal shield. (After mi * v ? , ' , , 



A. s. Woodward.) au, The Pteraspidae would seem to represent an ex- 

 impression of auditory treme stage in specialisation, in which the dorsal and 



organ (?) ; or, impres- -i -i -, -n 



sions of branchial ventral median plates, already present in Vrepanaspis, 



pEa?bod P yO.*'"' f r have extended and fused with neighbouring plates, the 



small intervening plates or scales having disappeared. 



Pteraspis, Kner ; Devonian, Europe. Palaeaspis (Holaspis), Clay p. ; 



Silurian, N. America ; Devonian, Europe. Cyathaspis, Lank. ; Silurian 



and Devonian of Europe. 



FIG. 172. 



Order 2. CEPHALASPIDOMORPHI (Osteostraci). 



Like the Coelolepidae, these Palaeozoic fish have an expanded 

 depressed head-region, which narrows behind to a heterocercal tail- 

 (Fig. 173). There is a single dorsal fin. The lateral edge of the 

 broad front region, which probably includes at least the most 

 anterior portion of the trunk, extends into a longitudinal flap in the 

 Ateleaspidae. This lateral expansion closely resembles the similar 

 flap in the Coelolepidae. It may represent a paired fin-fold, for in 

 the Cephalaspidae it is developed into a well-defined outstanding 

 scaly lobe with a narrow base (Fig. 173). 



The mouth was ventral, and what appears to be a ventral 

 cloacal slit has been described in Cephalaspis at the root of the tail 

 (Fatten [326]). Traces of six gill-pouches have been seen in 

 Cephalaspis (Jaekel [243]) on the under surface of the head-region; 

 a number of ventral paired openings in Tremataspis possibly indicate 

 branchial openings (Fig. 176) (Rohon [371]). 



