286 TRANSACTIONS OF THE AMERICAN PHILOSOPHICAL SOCIETY 



by heavy ridges above and behind the eyes and on the occipital surface, for these regions 

 serve for the insertion of two powerful muscles: first, the dorsolateral muscle, which, arising 

 from the superficial parts of the myomeres, has been compounded into a single muscle 

 inserting into the base of the tail; second, the deep vertebral muscles, which pass obliquely 

 downward and backward from the back of the occiput toward the anal fin. The figure of 

 this highly specialized fish thus particularly well illustrates something that is true of all 

 vertebrates, namely, that the back part of the skull serves as the anchor or pivot for two 

 sets of stresses, one coming from the locomotor apparatus behind and another from the 

 jaws and branchial apparatus in front and below the occiput. 



Anacanthus barbatus 



Fig. 164. Anacanthus barbatus. After Day. 



Anacanthus (Figs. 164, 165) carries the grotesque specializations of Aleutera to an amaz- 

 ing extreme. The tendency to secondary lengthening results in transforming an originally 

 deep fish almost into a tube-fish. 



Ostraciontids. — In the trunk-fish (Lactophrys) (Fig. 166) of the family Ostraciontidae 

 the external appearance is quite different from that of the balistid type, principally because 

 of the replacement of scales by a rigid body armor mostly formed of hexagonal plates, 

 which are to some extent foreshadowed in certain balistids. The spiny dorsal has been 

 eliminated and in this genus horns are added. The eyes are enlarged and raised, drawing 

 the frontals, sphenotics and pterotics up with them. The mouth has been brought down 

 to the ventral border. But when the armor is stripped off, the skull (Fig. 166) is seen to 

 be fundamentally identical with that of Balistes with but a few minor changes. The mouth 

 is now tilted partly downward, which has required the extension of the mesethmoid past 

 the quadrate-articular joint so as to form an abutment for the stout premaxillae. The 

 teeth are small and incisor-like, belonging to a number of successive sets; the maxillae are 

 firmly united with the premaxillse, the palatine is apparently absent. This powerful 

 nibbling mouth is supported above by the strong coalesced ethmoid and parasphenoid 

 and below by the rather delicate quadrate. The hyomandibular, essentially similar to 

 that of Balistes, is broadly expanded and meets the parasphenoid in a secondary sutural 

 contact. The opercular and especially the preopercular are reduced. The interopercular 

 retains its tracker-like character. The gill-chamber is very small. 



Triodonts. — In Triodon bursarius of the Indian Ocean we have an important structural 

 link between the balistid stock on the one hand and the puffers (tetraodonts) and porcupine- 

 fish (diodonts) on the other. As noted by Tate Regan (1929, p. 325), Triodon has an air- 

 sac or diverticulum of the oesophagus like the puffers and porcupine-fishes. But it retains 

 the pelvis which these others have lost and it "resembles the Balistidae in having the pelvis 

 a long movable bone that dilates the air-sac. It differs, however, in having no spinous 

 dorsal fin and in having the teeth represented by a beak." Here again it shows intermediate 

 conditions: in the upper beak, the two halves are separated by a median suture as in the 

 puflFers; in the lower jaw the two halves are united into a single beak like that of the 

 porcupine-fishes. 



