GREGORY: FISH SKULLS 295 



The fairly large skull is buried in the orbicular contour of the very short deep body. 

 In the front view the greatest diameter is above the relatively small eyes, which look 

 outward and slightly forward. The adipose rostrum forms a prominent rounded bump 

 above the very small thick-lipped mouth and thin, very sharp-edged, pointed beak. The 

 adductor muscles of the jaws are thick and fleshy with few or no tendons, and fill the 

 large space between the eye and the crescent-shaped suspensorium. The hyomandibular 

 is prolonged above and behind with the long horn-like projection of the pterotic, which in 

 turn is braced above by the large curved supracleithrum. Although the opercular is almost 

 vestigial. It is still functional, operated by broad fleshy dilatator and levator operculi 

 muscles. These muscles doubtless serve to expand the branchial chamber and draw in 

 water past the large oral valves. The large opercular flap has a small crease near the 

 posterior end, which enables this part to act as a hinge valve regulating the escape of 

 water from the opercular chamber. The six branchiostegals are likewise buried deep be- 

 neath the thick hide but are still freely movable. The subopercular is vestigial and the 

 interopercular reduced to a thread-like tracker that Is attached in front to the angle of the 

 mandible. 



The viscera are extremely voluminous, especially the liver, but as the body is narrow 

 transversely, the large kidneys have grown far forward and upward. The neurocranium, 

 as described by Steenstrup and Liitken Is largely cartilaginous. 



Thus may be traced the progressive stages of specialization from generalized percoid 

 fishes to deep-bodied nibblers, thence to long-faced balistoids, thence to secondarily short- 

 faced, large-beaked and low-headed plectognaths. The key to this transformation is that 

 the "habitus" of the remote ancestor becomes the "heritage" of the descendant after a 

 change of function and a change in the direction of evolution. 



In conclusion, when the advanced stage of specialization that is seen in Xesurus is 

 reached, one might well doubt the ability of Nature to produce viable creatures of any 

 greater degree of specialization. But Nature's limits are not so easily determined. Not 

 satisfied, as it were, with Xesurus, she next evolved the triggerfish {Balistes), going on to 

 Aleutera, which is a libel even on Balistes, and culminating in Jnacanthus, which is almost 

 a tube-fish in appearance. Returning to the pre-balistid model, she made some minor 

 changes and brought out the trunk-fish. Rising then to still more daring improvizations, 

 she invented the unique mechanisms of the puffers. But each new "invention" implies 

 also a further sacrifice of the capital stock of well tried, normal fish arrangements of the 

 earlier types, so that when Mola at last issues from Nature's experimental laboratory its 

 grotesque body might appear to the inexperienced to be fearfully handicapped by intensive 

 specialization. Ranzania is then brought forward, an elongated Mola, the latest but per- 

 haps not the last word in the evolution of the plectognaths. 



Postscript. — The otoliths of the order Plectognathi, as studied by G. Allen Frost (1930i, 

 p. 621), are "curiously aberrant in form showing little affinity with those of other orders." 



Allotriognathi (Opah, Oar-fish, etc.) 



In 1907 Tate Regan established the "Allotriognathi" as a suborder of the Teleostei, 

 to include such highly different looking forms as the orbicular Moonfish (Lampris tuna), 

 and the ribbon-like King-of-the-Herrings (Trachypterus) and Oar-fishes or Sea-Serpents 

 (Regalecus). I treat it here as one of the main divisions (suborders) of the Acanthopterygii 



