ACANTHODII. 37 



pectoral fin, either directly apposed at their base or united by a 

 small, interposed third element (x). The supporting arch is 

 complete. The greater portion of it consists of a pair of 

 vertically elongated basal elements (b), each having a straight 

 rod-like axis, filled with calcite in the fossils, and thus originally 

 either hollow or occupied by uncalcified tissue; behind this 



FIG. 29. FIG- 30. FIG. 31. 



Fio. 29. 



Acanthodes wardi ; skeletal parts of pectoral fin, about nat. size. U. Carboni- 

 ferous ; Staffordshire, b, hollow basal element; r, dermal rays (actino- 

 trichia) ; s, dermal spine forming anterior border of fin. (From Brit. 

 Mus. Catal.) 



FIG. 30. 



Diplacanthus striatus ; pectoral arch and fins, nat. size. L. Old Red Sandstone ; 

 N. Scotland. 5, hollow basal element, sometimes termed clavicle or 

 claviculoid, but probably homologous with (b) in Acanthodes (fig. 29) and 

 Parexus (fig. 31) ; cl., dermal plates, apparently clavicular ; x, hollow 

 skeletal base which may be homologous with the basal element (b) in 

 Acanthodes (fig. 29), but more probably results from the union of the 

 anterior () and posterior (m) fin-spines. (From -Brit. Hits. Catal.) 



FIG. 31. 



Parexus falcatus ; skeletal parts of pectoral fin, nat. size. L. Old Bed Sand- 

 stone ; Forfarshire. b, hollow basal element; cl., dermal plate, apparently 

 clavicular; *, dermal spine forming anterior border of fin. (From Brit. 

 Mus. Catal.) 



axis there is a thin laminar expansion of bone, diminishing 

 upwards, and apparently extending downwards and inwards to 

 form an inferior limb. The pair of large elements does not 

 meet in the median line below, but is separated by a much 

 smaller pair of lamina? (cl.), united in a finely dentated mesial 



