APPENDIX C. — REPTILES. 837 



from S. onaculatus, the chief character of which consists in the 

 truncated snout. The eyes are of medium size, situated behind 

 the angle of the mouth. Their position and size in S. maculatus 

 we do not know accurately, but in S. mexicanus these organs 

 would be considerably smaller and more approximated to the end 

 of the snout, judging of these peculiarities from the various 

 sketches given of that species, and proportionally much smaller 

 than in a specimen of Siredon in our possession from the city of 

 Mexico, and of about the same size as our S. lichenoides. If these 

 proportions are correct in the drawings of S, mexicanus^ we would 

 undoubtedly have a species which could not be accurately charac- 

 terized until further information should be obtained. The nostrils 

 are very small and near the end of the snout. The body is sub- 

 cylindrical, subfusiform, broader and deeper at its origin than on 

 any point backward. The tail is very much compressed, elongated. 

 and tapering into a point. The dorsal membrane commences at the 

 occiput, rising gradually until the middle of the tail, whence it di- 

 minishes again toward its pointed tip. The membrane under the tail 

 is lower than that above, extending from behind the vent to the tip 

 of the tail, and reaching its greatest height on its anterior third, but 

 diminishing more rapidly forward than backward. The anal open- 

 ing is very large, elongated, and rendered very conspicuous by the 

 great development of the fleshy masses which constitute its margin. 



The presence of four external flaps, provided with respiratory 

 fringes, is a generic character, belonging to all the species hitherto 

 known. Their real appearance has been misrepresented in many 

 sketches, as we could satisfy ourselves by the examination of two 

 species preserved in alcohol. The branchial fringes do not extend 

 all along the upper edge of the branchial flap. They occupy 

 densely the lower edge of that cutaneous appendage from its 

 origin to its tip, and thence for a short space above, but much 

 less developed here than below, as we have endeavoured to show 

 in the profile of fig. 2. The fringes themselves are very much 

 flattened, tapering, and disposed upon a double row, so that each 

 of them appears as if double ; but it is easy to ascertain that the 

 row on either side does not combine with the other. 



The fore and hind legs have nearly the same length when mea- 

 sured from their bases to their extremities ; the hind ones, however, 

 are much thicker, and the toes of both pairs are neither so slender 

 nor so elongated as in S. mexicanus and S. maculatus. 



The ground colour is blackish brown ; there are irregular patches 



22 



