Class Elasmobranchomorpha--sharks, rays, skates, chimaeras
     and Iniopterygians

     probably sister group to Osteichthys (Young 86, Money 84,
          Schaeffer 75), based on
          long palatoquadrate, expanded posteriorly
          teeth develop from dental lamina

     subclass Chondrichys--sharks, skates, and rays
          probably a natural group

     subclass Holocephali--chimaeras, ratfish
          uncertain affinities, Chondrichthys or Ptyctodonts

Chondrichthyan denticles (and fish scales) in Upper Silurian
good Chondrichthys first appear in late lower Devonian
     e.g., Cladodus M Dev --Perm; Cladoselachii Upper Devonian to
Pennsylvanian; Selachians with Ctenochanthus (Penn), Hybodus (Perm- Cret), Symmoriids (plesiomorphic group);

Characters:
no swimbladder
no internal or external bony skeleton (calc. cartilage)
heterocercal caudal?
paired fins P1 P2 D 1 or 2 (w spines after Paleozoic), A later, 
fins with basals and radials
oviparous or viviparous
claspers in males 
placoid teetn drrived from scales, dentine and enamel-like 
two ancient branches
     subterbranchialia
     (iniopterygia, chondrenchelyidae, helodontidae, menaspidae,
cochliodontidae, chimaridae, )

     Elasmobranchii 2 branches:

          Euselachii--

          (etc, Xenacanthida, cladoselachida, symmoriidae,
orodontids, petalodontids +_ orodontida

7 major Paleozoic groups
cladoselachida  USil Dev, Dspines, e.g. Cladoselache
symmorids--c symmetrical
Eugeniodontids--symphyseal whorl
Squatinactida--raylike
Petalodontids--shark like 
Xenacanthida--Dev Triass, 2 A fins
Ctenacanthida>>Hybodontids (&Neoselachii)?
mostly marine history