THE ELASMOBRANCH FISHES 9 



accomplished that on the border line between the two the characteristics of 

 the one often resemble those of the other so closely that it is difficult to say 

 this is a shark, that a ray. This will be made the more evident upon a consid- 

 eration of a series of these forms. 



Transitional Series 



In such a series, Carcharias, the "man-eater" (fig. 16) , may be taken as a highly 

 specialized type. The fusiform body, even more than that of Acanthias, is 

 fashioned for cleaving the water ; the caudal and pectoral fins are powerful ; 

 and the sharp pointed teeth are adapted for seizing and holding prey. In fact 

 every line of its structure is an index of predacious perfection. To a less extent 

 the same is true of Galeus. In Mustelus, although the body is highly special- 

 ized in this respect, the teeth, varying from the type, have generally become 

 broad and flattened for crushing. 



Heavier of body and clearly less graceful in form is the lamnoid type, 

 Lamna cornubica, in which, although the teeth are long and fang-like, the 

 body is relatively cumbersome and the pectorals are placed farther back. The 

 lamnoids, however, retain their restless nature and are classed among the 

 more predacious of the pelagic fishes. 



In Heterodontus (fig. 17) we see a still less graceful type. In it the pectorals 

 are expanded, suggestive of a less active nature, more given to foraging over 

 the ocean floor in search of a shellfish diet. Furthermore, in Heterodontus the 

 posterior teeth are of the crushing type adapted to such food (see p. 130, 

 fig. 128) . Yet it is worthy of notice that the most anterior of the teeth are pre- 

 hensile showing that the grasping of food is possible. 



Next in the series is Squotina, the angel fish (fig. 18), in which the body is 

 greatly flattened. Furthermore, the pectorals extend forward and the dorsal 

 fins have shifted to a posterior position. In all these respects Squatina is 

 ray-like. But the gill clefts, although covered with a flap, open laterally, and 

 other significant internal structures of fin and skull cling tenaciously to the 

 shark type. 



In Pristis, the sawfish (fig. 19), the anterior part of the body is still more 

 flattened; but the caudal region is like that of a shark. The gill clefts of Pristis, 

 however, are entirely ventral in position; and more important still its pec- 

 torals have fused to the sides of the head so that in essential respects it has 

 passed over the batoid line and is clearly among the rays. 



Slightly more ray-like is Rhiiiohatis product us, the guitar fish (fig. 7) . But 

 even here, although the head and body are depressed, the pectorals are rela- 

 tively small and the tail is still the organ of active propulsion. From this'shark- 

 like ray to others singularly flattened in form are rays in great variety. 

 ""^In the skate, Eaia erinacea (fig. 20), the pectorals extend forward to the 

 region of the nose, and the pelvics are large. Correlated with the greater devel- 

 opment of the paired fins the caudal region, including the dorsal fins, is poorly 

 developed, the dorsal fins liaving migrated still farther back on the tail. In this 



