4 PROF. HUXLEY ON THE SPECIFIC AND GENERIC 



Natterer* bas described tbree species of Caiman — G. pal/pebro- 

 sus, O. trigonatus, aud C. gibhiceps. Tbe Caimans abound chiefly 

 in tropical South America ; but they are found as far north as 

 Mexico, a specimen of C. palpehrosus in Dr. Grant's collection 

 coming from that country. 



Genus 3. J AC are. 



The snout is broad, and rounded at the endf- Each prefrontal 

 bone is traversed close to its anterior extremity by the ends of a 

 strong transverse ridge, which then curve round and pass forwards 

 on the lachrymal and maxillary bones, to subside opposite the 

 ninth tooth. The anterior nasal aperture is not divided by bone. 

 The vomers, separated by a longitudinal suture, appear in the 

 palate between the premaxillaries and the palatine plates of the 

 maxillaries. The temporal fossa), though not large, are open. The 

 webs of the feet are small. The dorsal scutes are articulated 

 together, as in the preceding genus ; and there are similarly-arti- 

 culated ventral scutes. There are 18-20 teeth on each side, above 

 and below ; and the fourth tooth in the upper jaw is the largest. 

 The mandibular symphysis extends back nearly to the fifth tooth. 



In a skull of Jaca/t^e (Jlssipes ?), 19 inches long, in the British 

 Museum, I find that part of the vomer wliich is visible in the 

 palate to be a rhomboidal plate, somewhat truncated anteriorly, and 

 rather more than 1\ inch long and 1 inch wide. Its anterior end 

 comes within |ths of an inch of the posterior margin of the an- 

 terior palatal foramen. Its posterior margin reaches to the level 

 of the eighth tooth. The visible portion of each vomer is onlv its 

 anterior end, which forms a thick and solid wedge-shaped plate, 

 broader in front than behind, and articulating by a rough anterior 

 and outer face with the premaxilla, by an obliquely ridged pos- 

 terior and outer face with the maxiUa, and by its internal face 

 with its fellow. Its upper, rounded surface projects but little 

 into the nasal passage. 2^ inches behind its anterior end, the 

 posterior and upper extremity of the vomer passes into a thin and 

 narrow plate of bone, whose plane is at first inclined at an angle 

 of 45° to that of the anterior part of the bone, but gradually 

 becomes vertical ; as it does so it deepens, until, 3 inches behind 



* " Beitrag ziir niihercn Kenntniss der Sudamerikanischen Alligatoren," 

 ' Annaleii des Wiener Mus.,' Band i. 



t According to Natterer, the dental formula of J. nigra and J. fissipes is 



-- — -, of J. sclerops — — — , of J. rallifrom and J. vunctulata ,_— r^. 



