322 



MEMOIRS OF THE CARNEGIE MUSEUM. 



Gymnarchus, through Mormyrus, up to the extremely specialized organ of Gnatho- 

 nemus. Progressive stages in the elongation of the rostrum in scombroid fishes 

 have been pointed out by Regan. The structural changes which have attended 

 the formation of a beak are illustrated in the following diagrams: 



Fig. 1. Diagram showing the structure of the rostrum in AcanUwcyhium (a), Histiophorus (6), 

 Xiphias (c), and Xiphiorhynchus (d). pmx, prffimaxillary; mx, maxillary; na, nasal; eth, ethmoid; fr, 

 frontal. (After C. T. Regan, Ann. & Mag. Nat. Hist. (8), Vol. Ill, 1909, p. 73). 



No attempt has ever been made, so far as the present writer is aware, to explain 

 by what means or processes the gradual formation of a rostral beak has been brought 

 about. An interesting theory, however, has been advanced by Dr. Gregory to 

 explain the progressive elongation of the pre-orbital region in the Syngnathidte to 

 form a tubiform snout with terminal jaws. The explanation given is as follows: 



