TBE ELASMOBRANCHS 55 



anaspids had a pair of pectoral spines, all of which 

 probably served as balancing and anchoring organs. 

 Though possibly of limited use for locomotion, these 

 spinous processes, developed as outgrowths of the heavy 

 body armor, were to supply the nodal points for the evo- 

 lution of the fins of the later fishes. 



None of the ostracoderms had movable jaws articu- 

 lated with the skull, for which reason they are placed 

 with the living cyclostomes (kyJdos = circle; stoma = 

 mouth) in the superclass Agnatha, or jawless vertebrates. 

 The mouth, as in the protovertebrate, consisted of a 

 round opening with hairlike appendages adapted for 

 straining microscopic animals and plants from off the 

 bottom of the streams. However, the armor around the 

 mouth necessarily took the form of movable plates, and 

 these plates afforded the raw materials for the later evo- 

 lution of a movable lower jaw hinged to the cranium, a 

 notable event because it set the pattern for the evolution 

 of all the later jaw-bearing vertebrates, collectively 

 known as the Gnathostomata (gnaihos = jaw; stoma = 

 mouth). 



The Silurian saw the transition from the bottom-living, 

 filter-mouthed ostracoderms to the free-swimming, jaw- 

 bearing fishes, and by the Devonian the vertebrates had 

 acquired the form of active, predatory fishes, the Placo- 

 dermi {plax = tablet or flat plate; derma = skin)— inci- 

 dentally the only one of the eight classes known to verte- 

 brate history which has become extinct. By the Middle 

 Devonian these placoderms were represented by several 

 orders (Figure 6). 



The Acanthodii (akantha = spine or thorn; odiosus = 

 hateful), or 'spiny sharks,' were covered with small, flat, 

 bony scales. They had one or two large dorsal spines and, 

 on the ventral side of the body, two symmetrical rows 

 of spines running from the pectoral region to the anus 

 and ranging in number from two to seven pairs. Though 

 these spines were probably not movable they carried at- 



