26 PEOr. HUXLEY ON THE SPECIFIC AND GENEEIC 



vent, corresponded exactly with the twenty-first dorsal row. It 

 was clear, therefore, that three ventral rows were interpolated 

 somewhere between the twelfth and twenty-first dorsal rows ; and 

 on close examination I found this interpolation to arise from the 

 doubling of the fourteenth, fifteenth, and sixteenth ventral rows. 



I have examined Jacare Jlssipes and nigra, Caiman trigonatus, 

 and C. gihhic&ps, in the British Museum ; and I find, in all, dorsal 

 and ventral armour having the same essential arrangement as 

 that just described. A specimen of Caiman palpehrostis about 

 two feet long, the opportunity of examining which I owe to Dr. 

 Grant, exhibits the dorsal and ventral shields (whose scutes are in 

 the main similarly arranged) very beautifully ; and a young Jacare 

 of about 18 inches in length, for which I am indebted to the kind- 

 ness of the same gentleman, proves that the scutes are developed 

 even in specimens of this age. I have no hesitation therefore in 

 expressing my belief that this singularly complete dermal armour 

 will be found to be characteristic of all the species of the genera 

 Caiman and Jacare. On the other hand, I have examined Alli- 

 gator Mississipiensis, Crocodilus vulgaris, C. biporcatus, G. Ameri- 

 canus, C. rhombifer, and C. iomhifrons, Mecistops cataphr actus, 

 and Gavialis Gangeticus, of various ages and sizes, without having 

 been able to discover a trace of ventral scutes. This is the more 

 remarkable, as the well-marked ventral and dorsal shields of many 

 of the ancient Teleosauria would lead one to expect a correspond- 

 ing exoskeleton (if anywhere) in their nearest allies, the modern 

 Gavialidce. However, Goniopholis, with its strong armour, is 

 more like an ordinary Crocodile ; and I have recently discovered 

 that a true Crocodile in some respects curiously similar to C. hom- 

 hifrons (C. Hastingsice) was covered with scutes exceedingly like 

 those of the modern Caiman and Jacare. 



In minute structure the bony scutes of Jacare closely re- 

 semble those of such a fish as a Sturgeon : a middle layer, con- 

 taining so many canals as to appear almost cancellated in longi- 

 tudinal or transverse section, is covered externally by a thin, and 

 internally by a thick, layer composed of bony lamellae, nearly par- 

 allel to the plane of the scute. E-ound the canals of the middle 

 layer, the bony lamellae are disposed concentrically, to a greater or 

 less extent. The lacunae are of very various shapes ; and there are 

 perhaps as many short as elongated forms. The canals of the 

 middle layer communicate by large branches with the inner, by 

 smaller and fewer branches with the outer surface of the scute. 



In the young Jacare mentioned above, I found the dermis to be 



