2 PROCEEDINGS OF THE NATIONAL MUSEUM. tot,. 66. 



13-15) teeth aw therefore e.sseiitially alike in form and structure. 

 In Vicygna the permanent teeth (figs. 5-9) are strikingly different 

 from their predece.ssors (figs. 1-3). The milk teeth are more elon- 

 gate, llian those of either adult or young Lama (greatest hreadth 

 about one-sixth or less of the greatest length), but their form is still 

 obviously cuneate; the bases, liowei^er, remain open, and there is no 

 enamel on the lingual side of the crown. The enamel of the labial 

 .side occupies slightly more than half the length of the tooth, a con- 

 <lJtion intermediate between that which is seen to occur in the milk 

 and permanent teeth of Lama. The adult teeth of Vicugna have lost 

 all trace of the cuneate form. They are parallel-sided, fully ten 

 times as long as Avide, armed with a rodent-like plate of enamel 

 confined to the lingual aspect of the tooth and extending to within 

 2 or 3 millimeters of the widely open base. Apparently these teeth 

 continue to grow through most of the animal's life; but in extreme 

 senility (in a captive individual at least) growth may cease and the 

 teeth may become completely worn down to stubs (fig. 4). 



Comparison of the figures here published with Figure 8 of Doctor 

 Andrews's Plate ^0 v. ill show the striking likeness which exists be- 

 tween the teeth of Vicugna, and Myotragus. Aparently it is not defi- 

 nitely known whether the incisors of the goat were truly ever-grow- 

 ing as they are in rodents or whether they exhibit the same conditions 

 with regard to maner of growth as those f(mnd in the vicimia. As- 

 suming that they were strictly rodent-like in this respect they would 

 represent a stage of development a step farther advanced than that 

 exemplified by the adult incisors of Vicmgna. The transitional condi- 

 tions leading back from the structure present in the adult vicunia to 

 the one normal to the incisors of artiodactyls in general may be seen 

 in the vicunia's milk teeth. Here {\\q original cuneate form has be- 

 come elongated, the base of the root has been permanently opened, and 

 the enamel has been eliminated from the lingual aspect of the crown. 

 '\^niile the morphological elements of the problem of the development 

 of rodent-like incisors in artiodactyls therefore no longer present 

 any special obscurities the physiological impulse which may have 

 initiated the change of form in the teeth of both the vicunia and the 

 Balearic goat api)ears to be still entirely unknovrn. 



EXl'LANATION OF I'LATp]. 



Incisor teeth (if Mcnniii and (iminiu-o. AU ti.iiuies siiiclitly rcdiK-eil. 



Viciajyiu ricufiiui. 



Fio. 1. No. ;iS4r)l. Milk dentition, i, left, lingual surface. 

 Milk dentition, ii left, lateral surface. 

 Milk dentition, i, right, labial surface. 

 Completely worn out stubs of li, right and ii left, in 

 senile captive individual. 

 No. 194297. I'ermanent dentition, i^ left, lingual surface. 



