G8 
MR. W. K. PARKER OK THE STRUCTURE AXD 
can be seen with its thickened sides, growing backwards as rudimentary “ tympanic 
wings,” and wedging forwards under the uniting orbitosphenoids. The distal part 
of the alisphenoids comes into view outside the pterygoids, and the foramen ovale, 
foramen rotundum, and common passage for the orbital and optic nerves (V 3 ., V 3 ., 
V 1 ., II.) can all be distinguished in this aspect of the skull. 
Here the large unique pneumatic pterygoids show their communicating passage 
with the great naso-palatine canal, and therefore, also, with the whole system of 
air-galleries in the nasal labyrinth. 
I have mentioned the parallelism of the Unau and the Mole in the peculiar 
temporary pretympanic process of the malleus ; here in the Ai (Plate 13, figs. 8, 9), 
the outer and inner aspects of the tympanic annulus are almost precisely like those 
of the Hedgehog, and of the young Mole; they have a considerable concavity, a large 
amount of ossified floor, and strong, incurved cornua, the front horn being deeply and 
obliquely notched for the processus gracilis. 
Myrmecophagid,e. 
Young of Cycloturus didactylus; one with head 1 ^ inch long, and another 4§ inches 
long from snout to root of tail, and head 1 ^ inch long. 
I shall treat these as one stage ; they correspond well with the larger specimens of 
the Armadillos, and with the third stage of the Sloth. This small arboreal Anteater 
comes in well after the Sloths; the difficulty of comparison of these two Families is 
lessened considerably hi this case. 
Once well understood, the same stage in each type— Sloth and Anteater—can be 
put side by side, and then the special adaptive modifications may be accounted for and 
the true radical kinship of these two, apparently so diverse, forms, can be demon¬ 
strated. I have found that by putting them alongside of each other, and carefully 
removing them special “ marks,” the evidence for a common descent is very great. We 
have a similar comparison to make between the toothed and toothless Anteaters of the 
Old World— Orycteropus and Manis —and also, indeed, between the Ornithorhynchus 
with its horny “ succedanea ” for teeth, and the absolutely edentulous Echidna. 
Nowhere can the modifications produced by the gradual abortion, and the complete 
suppression, of teeth in the higher Yertebrata be better seen than in the Edentata or 
Bruta. In the Chelonia the massive horn-covered jaws are less modified than might 
have been expected ; the strong shears that serve as a makeshift for teeth, ask for as 
strong a setting as the normal teeth of a Reptile. 
So also hi Bmds, to a greater or less degree, as may be seen by comparing the skulls 
of the toothed Bmds (so ably described and exquisitely figured by Dr. Marsh) with 
those of the existing toothless types. 
But, in the Mammalia, we are so familiar with the huge dental apparatus seen in 
this or that high Eutherian—jaws answering to teeth, and teeth to jaws—that any 
