12 AGNATHA. 



known, differ from the Cephalaspidae in the circumstance, that 

 their interorbital piece is loose and often lost in the fossils. 

 Tremataspis schrenki and other species occur in the Upper 

 Silurian of Oesel. 



ORDER 3. ANTIARCHI. 



The dermal skeleton of the Antiarchi is much more complex 

 than that of the two preceding orders. The bone is dense, 

 though with vascular cancelte in the middle layer of the more 

 thickened plates ; the sensory canals traverse the armour along 

 well-defined superficial grooves. The external ornament always 

 consists of tubercles and coarse rugae. A pair of armoured 

 appendages, usually moveable, is fixed to the antero-lateral 

 angle of the body-shield, which is always moveably articulated 

 with the head-shield. A description of the best-known genera, 

 Pterichthys and Bothriolepis, will serve to enumerate the 

 principal features of the group. 



Pterichthys (fig. 12). The whole of the armour is composed of over- 

 lapping plates, which exhibit a bilaterally-symmetrical arrangement. The 

 cranial shield is small compared with the armour of the trunk, and is 

 confined to the dorsal and lateral aspects. The eyes (orb.) are placed 

 close together near the middle, and the quadrangular plate between them 

 is loose. This plate is marked by a deep pit on its visceral aspect, 

 perhaps for the reception of a pineal body, and is supplemented by 

 another narrow element in front. A thin oval ossification in the posi- 

 tion of each eye, appears to represent a partially hardened sclerotic. On 

 either side of the head-shield there is a loose plate (op.), which seems to 

 have formed an operculum, for its posterior margin was evidently free, 

 while its anterior strongly convex margin is notched in such a manner 

 as to suggest the ordinary articulation of a fish-operculum. Below the 

 head-shield in front of the position which must have been occupied by 

 the mouth, there is also a pair of loosely attached plates (m.), which meet in 

 the middle line and are much broader than long. Each is notched at its 

 postero-external angle, as if for the openings of a paired nasal organ? 

 Though the head is moveably articulated with the trunk, there are no 

 ginglymoid processes or surfaces. The body-armour surrounds the trunk 

 completely and extends backwards for a very considerable distance, but 

 does not include the anus. Its ventral surface is flattened, while the 

 dorsal shield is much arched. Near their front extremity the ventro- 

 lateral plates are strengthened by a robust transverse ridge on the vis- 

 ceral aspect, and close to this the pectoral appendages are fixed by a 



