HETEROSTRACI. 5 



there occurs a regular series of small indentations, to the 

 number of five or six, which suggest the original presence of 

 well-separated gill-pouches ; and these would have opened into 

 an external chamber under the shield, with an outlet near the 

 postero-lateral angle of the armour. 



The simplest Ostracoderms (HETEROSTRACI) occur in the 

 Upper Silurian and Lower Devonian, and exhibit no bone- 

 cells in any part of their dermal armour ; the second order 

 (OsTEOSTRACi) is confined as a rule to the same horizons, 

 though also rarely met with in the Upper Devonian ; the third 

 order (ANTIARCHI), with a complex of dermal plates and a 

 remarkable pair of appendages, is essentially Devonian, and as 

 abundant in the uppermost, as in the lowest strata. Nearly 

 all the genera mimic in a curious manner the contemporaneous 

 Eurypterids. 



ORDER 1. HETEROSTRACI. 



The structure of the hard shield in the Pteraspidians, or 

 Heterostraci, is unique among the Chordata. Each plate 

 consists of three superposed layers an inner " nacreous " layer 

 of lamellae, a relatively thick middle zone of polygonal cancellae, 

 and an outer hard layer of vaso-dentine. Dermal sense-organs 

 are well-developed and arranged in canals, which traverse the 

 middle layer and open externally by a double series of pores. 

 The dorsal shield comprises few pieces, firmly united in the 

 adult; and the orbits are wide apart, being laterally placed. 

 The ventral shield is in a single piece (named Scaphaspis). 

 Paired appendages appear to be absent. 



The only known family of Heterostraci is that of the 

 Pteraspidae, in which the external layer of each dermal plate 

 forms an ornament of very fine, concentric, closely arranged 

 ridges, parallel with the outer margin. The rostral region is 

 relatively small. The scales of the caudal region, when present, 

 are numerous and rhomboidal. Three genera are distinguished 

 according to the complexity of the dorsal shield. 



Cyathaspis (figs. 2, 3). Known only by the dorsal and ventral shields, 

 which have sometimes been found in natural association. The dorsal 

 shield (fig. 3) is oval, consisting of four separately calcified portions a 

 large central disc (d), with a short azygous rostral plate (r) anteriorly, 



