126 ON THE BILL OF THE HERON. 



of softer and finely wrinkled skin between the eye and the nostril 

 divide the covering of the upper jaw into a dorsal and a paired 

 lateral plate. The covering of the mandibular rami is marked by 

 the presence of some parallel oblique ridges ; these are jjerhaps 

 still more conspicuous just at the time of hatching, but quickly 

 disappear afterwards (tig. 2). 



One day after hatching (length of gape 25 mm.) the bill is 

 longer and conspicuously thicker, and the swelling of the tip has 

 almost disappeared. The covering of the tip of the upper jaw is 

 still marked off by its texture and relief, and the tip-covering of 

 the mandible is marked off by two irregular crack-like furrows 

 as well as by its solid texture. The division between the upper and 

 lateral plates of the upper mandible is plainly visible (fig. 3). 



About three days after hatching (length of gape 32 mm.) the bill 

 has nearly assumed its conical shape, but is still relatively short 

 and thick and the tip is still conspicuously hooked. The horny 

 covering of the mandible-tip is still marked off by irregular 

 furrows ; in the upper jaw it is no more distinct ; the lateral 

 furrows before the nostril, dividing the upper and paired lateral 

 horny plates, are still clearly visible (fig. 4). 



These changes seem to deserve attention on more than one 

 ground. The simple rhamphotheca proves to be only a late stage 

 of the compound one, as very clear vestiges of separate pieces are 

 visible in younger stages. In the upper jaw we find a tip piece, 

 a dorsal piece, and a paired lateral piece ; in the mandible there 

 are a tip piece and a paired lateral piece. The form of the Ardeine 

 bill proves to be a derivative one. The presence of the tip-hook 

 in the nearest relatives of the Herons, Scopus and Balcenicejjs, is 

 hence a primitive feature. In younger stages the shajDe of the 

 bill, combined with its compound rhamphotheca, recalls very 

 nearly that of a Cormorant, especially of a young one about the 

 time of hatching, and also of the nestling of a Frigate-bird, and 

 of a Pelican about the time of hatching. This points once more 

 to the affinities of the Ardese and Steganopodes. The change in 

 shape of the bill in the Ardeinpe is obviously due to their habits 

 of spearing their prey instead of snatching as most of the Stega- 

 nopodes do. The peculiar heron- like shape of the bill in the 

 Darter [Plotus anhinga) is connected with the same habits, and 

 the likeness of the bill's shape in the Herons and Plotus is to be 

 considered as a case of parallel evolution. The transitory ridges 

 on the mandibular rami recall remotel}^ — by their direction also 

 — the structures so much developed in the Anseres, aiid also, 

 perhaps, the indentations of the bill in the fossil Odontopteryx. 



EXPLANATION OF PLATE XIII. 



Fig. 1. Bill and liead of an embryo of Ardea cinerea, length of gape 12 nam. 

 1X2. 



2. The same, one day before hatching. IXI'75. 



3. Young bird, one day after hatching. IXl'o. 



4. The same, about three days after hatching. IXl'o. 



