THE GANOIDEI. 147 



the oldest fish at present known. The head is covered by 

 a continuous shield, which has the structure of true bone, 

 in Cephalaspis, but more resembles certain piscine scales, 

 in Pterapsis. The shield is prolonged into two horns 

 at its posterolateral angles, and a median dorsal back- 

 ward prolongation usually bears a spine, in Cephalaspis ; 

 the body is covered with flat bony scales or plates, and 

 possesses two large pectoral fins. The characters of the 

 body and fins of Pterapsis are unknown. JSTotwith stand- 

 ing the excellent presei*vation of many of the specimens of 

 these fishes, they have, as yet, yielded no evidence of jaws 

 or teeth. Should jaws be absent, the Gephalaspidce would 

 approach the MarsipvbrancMi more nearly than any of 

 the other amphirhine fishes do. 



6. The Placoclermi, comprising the genera Coccosteus, 

 PtericMhys, Asterolepis, and some others, are known to occur 

 only in the Devonian and Carboniferous formations. In 

 these fishes the pectoral region of the body is incased in 

 great bony plates, which, like those of the skuU, are orna- 

 mented with dots of enamel. The caudal region was covered 

 with small scales in PtericMhys, while iu Coccosteus it appears 

 to have been naked. The pectoral member of PtericMhys is 

 exceedingly long, covered with suturaUy-united bony plates, 

 and united with the thoracic plates by a regular joint. In 

 Coccosteus the pectoral member seems to have had the ordi- 

 nary construction. The bones of the head and thorax of 

 Coccosteus nearly resemble those of certain Sduroid fishes 

 (e.g. Clarias) in their form and arrangement, and it seems 

 probable that the Placoclermi were annectent forms between 

 the physostome Teleostei and the Ganoidei. 



7. The Acanthoclidce, on the other hand, seem to have 

 connected the Ganoidei with the Elasmobranchii. The scales 

 of these fishes of the Devonian and Carboniferous formations 

 are very small, and similar to shagreen ; spines, resembling 

 the dermal defences of the Elasmohrancliii, are placed in 

 front of more, or fewer, of the median and of the paii-ed 

 fins. The skull appears to have been unossified, and the 

 pectoral arch seems to have consisted of a single bony hoop. 



