102 TRANSACTIONS OF THE AMERICAN PHILOSOPHICAL SOCIETY 



eyes and olfactory capsules out to the edges of the suddenly projecting shelves on either 

 side of the head. This flattened face serves apparently as a bow rudder and makes possible 

 very quick turns in pursuit of fish (Nichols and Murphy, 1916). 



The adaptive radiation of the teeth and jaws in the elasmobranchs is very wide, so 

 there are naturally marked differences in the neurocranium. At one extreme we have the 

 highly predaceous, almost snake-like jaws of Chlamydoselachus anguineus, in which the 

 long slender jaws are produced backward; the hyomandibular is consequently elongate and 

 inclined backward with a corresponding loss of the contact between the otic region and the 

 palatoquadrate. This is the specialized hyostylic type. The neurocranium is cartilaginous 

 and relatively delicate. 



At the other extreme of the true sharks we have the massive short jaws and short 

 thick neurocranium of the Port Jackson shark {Cestracion or Heterodontus). The suspen- 

 sorium remains hyostylic but the palatobasal contact is widened. The massive whorls of 

 flattened teeth borne by the heavy jaws seem well adapted for the crushing of mollusc 

 shells and crustaceans. Between these extremes stand the moderate to small-sized jaws 

 of the squaloid sharks, beset with rows of small sharp-edged teeth adapted for biting. The 

 suspensorium is still hyostylic. 



The rostrum of primitive sharks is prolonged as a bluntly-rounded prominence in front 

 of the olfactory capsules, perhaps to improve the stream-lining of the head as a whole, 

 perhaps to afford an expanded area for the ampullse of Lorenzini and other sensory organs. 

 Primitively the skeleton of the rostrum was probably continuous (as in the Permian 

 pleuracanths), but in typical sharks it has become fenestrated and reduced to one median 

 and two lateral bars. The variations in the rostral skeleton have been used by Tate Regan 

 (1906a) in classifying the families of elasmobranchs. In the rays the anterointernal edge 

 of the pectoral fins is tied to the side of the rostrum and in the manta-rays the rostrum caves 

 in in front, while long projections of the pectoral fins, which have a sharp elbow and a 

 spatulate base, serve perhaps as oral palps or as funnels to direct the food into the mouth 

 when the fish is moving forward. 



The skates and rays show a remarkable progressive development of various types of 

 dental mills. In the rhinobatids the individual teeth are still distinct but the crowns are 

 flattened into more or less rhombic bases and the teeth form a rounded, indented whorl on 

 the border of the upper and lower jaws. In the higher rays individual teeth are replaced 

 by multitudes of minute vertically-placed fused rods, grouped into polygonal and broad 

 rectilinear blocks. The rectilinear series forms an anteroposteriorly elongate, convex 

 tract, or lower millstone, which opposes a similar plate on the broad palatoquadrate. The 

 latter is merely appressed against the wide floor of the neurocranium but is supported, 

 apart from ligaments, only by the stout hyomandibular. The ceratohyal is widely sep- 

 arated from the Meckel's cartilage, whereas in normal hyostylic elasmobranchs it is tied 

 by ligament to that cartilage. Hence the word euhyostylic was coined to describe the 

 suspensorium of the jaw in the rays (Gregory, 1904, p. 58). 



The jaws of all elasmobranchs differ from those of teleostomes in that there are no 

 dentigerOus dentary, coronoid nor splenial bones in the lower, and no dentigerous premaxillae, 

 maxillae, vomerine, palatal nor pterygoids in the upper jaw. The view that the labial car- 

 tilages of sharks represent the cores of the premaxillae and maxillae of teleostomes seems to 

 have nothing in its favor but the general topographic correspondence of these two sets of 

 elements in two widely separated groups. 



