iv. 2 o CEPHALASPIDS 127 



semicircular canals. Long tubes leading through the shield contained 

 the cranial nerves, which can be reconstructed in detail (Fig. 85). 

 On the under side of the shield is a series of ridges, which outline a 

 set of ten pairs of branchial pouches. The first of these lies far forward 

 at the sides of the mouth and the ridge in front of it is probably the 

 premandibular arch; it carries the profundus nerve (p. 152), which 

 was large. The ventral surface of the head was flat and covered with 

 small scales. Probably the gills were pouches, as in lampreys. The 

 canals of the aorta, epibranchial arteries, and some features of the veins 

 and heart have been preserved. 



The mouth was a slit at the extreme front end with which the 

 animals may have scooped decaying matter from the lake floor. 

 On the dorsal surface there are sunken areas, covered by small scales, 

 known as the median and lateral fields, and supposed by some to have 

 contained electric organs. They were apparently served by a very rich 

 blood-supply and a system of wide canals leads to the vestibular 

 region. These canals might have contained nerves, but Watson makes 

 the far more likely suggestion that they housed tubular extensions of 

 the labyrinth and served to carry pressure waves to the ear, perhaps 

 providing a substitute reinforcement for the defective lateral line 

 system. 



We therefore know in some respects as much about these fossils as 

 of many living fishes. They show in the complete segmentation of the 

 head the most primitive condition known among craniates. Many of 

 their features are very like those of modern lampreys and there can be 

 little doubt that, as Stensio suggests, the latter represent their sur- 

 viving descendants, which have lost the bony shield. 



The Anaspida (mostly Silurian) are placed by Stensio near the 

 Cephalaspids but they are less well known. They were small fishes 

 (up to 7 in. in length) covered with rows of bony scales (Fig. 87). The 

 tail shows a lower lobe larger than the upper ('hypocercal'). This 

 would presumably serve to drive the head end upwards perhaps to 

 compensate for the weight of its armour. The opposite ('heterocercal') 

 condition, found in cephalaspids and many modern fishes (for 

 instance, the dogfish), produces a tendency to negative pitch and is 

 associated with the presence of pectoral fins (p. 136). The anaspids 

 possessed a curious ventral or ventro-lateral fin fold (Fig. 87) or 

 perhaps a series of them. There were large paired eyes, median holes 

 presumed to be nasal and pineal and a series of up to fifteen small 

 round gill openings. 



We may consider here the fossil Jamoytius from the Silurian. The 



