155 



blance and of difference no doubt may be traced through the entire 

 structure ; as, for instance, the femur of the Proboscidia, although it 

 wants the third trochanter, so characteristic of the more typical Peris- 

 sodactyla, resembles the corresponding bone in that group in the cha- 

 racters of the posterior side of its upper part. 



If it be admitted that this assemblage of singularly modified forms 

 have sufficient resemblance to form a group which shall, with the 

 more typical Perissodactyla, constitute two divisions, about equal in 

 rank to the two divisions" of the Artiodactyla, there cannot be much 

 difference in opinion as to the manner in which this group should be 

 subdivided into families. The Proboscidia stand forth as one {Ele- 

 phantidce), and the Sirenia as another {Manatidce) ; while the Toxo- 

 don, which in its most essential characters seems to agree with both, 

 and in some points with each, has so many peculiarities of its owa, 

 that it appears entitled to rank as a distinct family of itself, which 

 should be placed between the other two, not as a " connecting hnk," 

 which its marked differences from either must forbid, and which if it 

 were, it would but annihilate the distinction that exists. 



It seems time that naturalists should have decided what it is that 

 constitutes an affinity ; whether a form can really be allied to several 

 widely-different groups. We may naturally expect to find, that 

 amidst the varied forms each part assumes, a character which is the 

 rule among the members of one group may be the exception in an- 

 other, vnthout of necessity supposing that a species presenting such 

 a character can truly belong to both, and thus tend to destroy the 

 difference of the original models on which the two groups are or- 

 ganized. In the present case, notwithstanding the peculiarities of 

 structure mentioned as connecting the Toxodon with the Rodentia, 

 its renowned describer, even while strengthening the idea of that affi- 

 nity by adverting to Cuvier's assertion that the Elephants approach 

 the same order, yet places it, apparently without a doubt, among the 

 Ungulata, to which it obviously belongs. Although Cuvier affirms, 

 that if all the parts of the head of the Elephant be compared succes- 

 sively with those of other animals, it is almost always among the Ro- 

 dentia that their analogies will be found, he alludes only to three 

 parts as indicating any such affinity. The relative size of the incisors 

 and their alveoli can" signify but little when their widely different 

 structure is considered ; and he correctly tells us why the infraorbital 

 foramen is large in both : the character of the os malre is common 

 to the Bats and Insectivora as well as the Rodentia, and seems to be 

 a frequent concomitant of a degree of organization comparatively low. 

 The direction of the incisors in the Toxodon differs very little from 

 that which we find in many of the typical Perissodactyla, and the 

 absence of roots is simply a physiological adaptation, and an indu- 

 bitable proof that the detrition" to which they were subjected was 

 considerable ; while on the other hand, the whole structure of the 

 cranium is on the ungulate type, especially different from the Capy- 

 bara and the forms allied to it, whose skulls present so many striking 

 characters, that if any resemblance really did exist, an anatomist to 

 whom they were familiar would certainly perceive it at a glance. 



