VI HYLIDAE 203 



of thin and sliglitly elevated ridge or fold of the skin extends on 

 to the neck. The suggestion, that this seam is burst open, in 

 order to set the full-grown young free, instead of their passing 

 through the existing opening, is scarcely credible. 



These Neotropical tree-frogs seem to be rare, and females with 

 embryos are of course still more uncommon, so that the best 

 account of their structure is still that given by Weinland ^ of K 

 oviferum. How the eggs get into the pouch has not yet been 

 observed, but it is most likely with the help of the male, im- 

 mediately after -fertilisation. The pouch forms two blind sacs 

 which extend forwards over the sides of the back. The eggs are 

 large, 1 cm. in diameter, and the enclosed embryos, or rather 

 tadpoles, had a length of 1 5 mm., with a large amount of yolk still 

 contained in the spirally wound intestine. The first two gill-arches 

 carried each a double thread, which expanded into a funnel- 

 shaped membrane, not unlike the flower of a Convolvulus, and 

 furnished with a capillary network ; the stalk contained muscular 

 fibres. These most peculiar structures are of course the much 

 modified external gills. Those of JV. testudineum and J\\ cornutum 

 are likewise bell-shaped. 



Hylella differs from Hyla chiefly by the absence of vomerine 

 teeth, and consists of about half-a-dozen small species, about one 

 inch in length. The fact that two species live in Queensland 

 and New Guinea, while the others are natives of tropical America, 

 suggests that this genus is not a natural but an artificial 

 asseml)ly, an instance of convergent evolution. 



PltyUo medusa, composed of about one dozen species of tree-frogs, 

 is characterised by the vertically contracted pupil, large adhesive 

 discs, and the opposable nature of the inner finger and of the 

 hallux, the last joints of which are like thumbs. The sacral 

 diapophyses are strongly dilated. The range of the genus 

 extends from tropical Central America to Buenos Aires. Most 

 of the species are about 2 inches in length, blue -green to 

 violet above, with white purple-edged patches on the sides of the 

 body ; the under parts uniform white, or with purple or brown 

 patches. The male has a subgular vocal sac. Some have more 

 or less distinct parotoid glands. Ph. dacnicolor of Mexico is 

 uniform green above, whitish below, and attains a size of more 

 than 3 inches. In Fh. hicolur of Brazil, the skin of the upper 

 ' Arch. Anat. unci Phys. 1854, p. 449. Also Boulenger, P.Z.S. 1898, p. 107. 



