DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 
7 
Dasypodid^e. 
First Stage. Embryos of Tatusia hybrida, If inch long, measured, in these, and 
the rest, from snout to root of tail (see Plate 1, figs. 7, 8). 
Second Stage. Embryos of Tatusia hybrida, 2 inches long. 
Third Stage. Embryos of Tatusia hybrida, 3 inches long. 
Fourth Stage. Embryos of Tatusia, peba, 3 inches long. 
Fifth Stage. Embryo (ripe) of Tatusia hybrida, 4 inches long. Ripe young of 
Dasypus villosus, 3f inches long. 
Sixth Stage. Young (new born ?) of Tatusia peha^ 4-| inches long. 
First Stage. —Tatusia hybrida (if inch long). 
In this, the youngest specimen obtained, I shall describe the chondrocranium with 
its commencing bony centres and visceral arches, as seen in dissection, and then give 
its structure in detail, as illustrated by a series of vertically transverse (transparent) 
sections. 
Afterwards, in the second stage, the investing bones will be described in their form, 
as well as in their relations. 
General remarks upon the early skull oj the Mammal. 
So great is the uniformity of the early chondrocranium in the Eutheria, or Placental 
Mammals, that this drawing (Plate 2, fig. 1), made from the skull of an outlying and 
low type, might serve as a diagram wherewith to illustrate the skull, at this stage, of 
the types of this Order, and of all the Orders above it. 
The figure of a chondrocranium like this, but a little less advanced, before the 
osseous centres have commenced in it—that of the Mole—will be given in my next 
paper; and such a skull comes very near to that of a Shark or, still better, of a Skate. 
The parts, or rather regions, of which it is composed, correspond very exactly with 
what is seen in those generalised, but not low, Fishes; and in this specimen, with 
bony centres appearing, the level is obtained which is permanent in the skull of the 
Dipnoi, and of such a low Ganoid as the Paddle-fish ( Polyodon ).'“' 
As in Cartilaginous Fishes and Amphibians, the chondrocranium may be compared 
to a basin or a boat, the upper part being unfinished, leaving a membranous fontanelle 
of greater or less extent; this is only partially filled in, at present, by tbe investing 
bones,, the frontals and parietals (f,p-)- 
The outline of this sectional view is very elegant, and quite similar to that of 
a vertical section of a Bird’s skull at a like stage, except that the nasal roof-cartilages 
*-See Bridge, “ On the Skull of Polyodon folium," Phil. Trans., 1878, Plates 55-57, pp. 683-733. 
