DEVELOPMENT OF THE SKULL IN THE MAMMALITA. t5 
throat. In the Skate (Pristiwrus, see Trans. Zool. Soc., vol. x., plate 35, fig. 4) all 
the pharyngobranchials are developed (chondrified) independently of the rest of the 
arches to which they belong, which for the most part undergo secondary segmentation 
into more or fewer joints. 
Here the segmentation of that part of the arch which in Fishes carries the gills 
is more than normal; the feebly developed epihyal (¢.hy.) is in two imperfectly 
divided joints, and the strong ceratohyal (c.iy.), which is imperfectly segmented 
from the lower epihyal, is completely cut across at its lower third. 
Below this part there is the normal (ichthyic) hypohyal (/.hy.), the shortest of the 
segments. * 
The basal part is a trifoliate basi-hyobranchial (b./.b7.), to the fore angles of which 
the hypohyals are articulated, and which gives off a pair of hypobranchial rudiments, 
and a free retral rudiment of the long, seemented bastbranchial bar of Fishes. 
I found only one inner segment of the opercular growth round the first cleft; the 
“ concha auris” becomes tubular in the meatus externus, and partly segmented ; the 
innermost segment in this case forms an imperfect ring, and its softish cartilage is 
rapidly becoming ossified to form the normal annulus tympanicus (a.ty ), in a groove 
of which Mecxev’s cartilage lies at its proximal end. The upper end is most ossified, 
the membrana tympani (7.ty.) 1s inserted into its inner face, and the radiating fibres 
of this membrane start from the internal angular process of the primary mandible 
(the manubrium mallei). The tensor tympani muscle (¢.¢.7.) is manifestly the counter- 
part of one of the “adductor muscles” of the hinder part of the mandible of a 
Reptile or Bird. 
This muscle, and the “ stapedius,” want tracing downwards; they are the accurate 
muscular correlates of the curiously specialised skeletal elements of this part of the 
skull. 
Vertically-transverse sections of the head of Tatusia hybrida.—First Stage. 
The figures given in Plates 3, 4, and the following descriptions must be compared 
with the figures and descriptions of the bisected and dissected chondrocranium ; the 
latter is of a later stage (Plate 5, fig. 1). 
Before going into details, I may remark that the huge nasal capsules that take up 
half the sections selected to be figured lie under the fore part of the skull, so that the 
floor on which the olfactory lobes le is the roof of the hinder and more important 
part of the nasal labyrinth. The septum between the two halves of this labyrinth is 
mainly formed of the intertrabecula, but the thicker part of the septum, where it is 
becoming lower behind, has the cornua trabeculze confluent with the azygous bar. 
, 
* Tn this, as in my former papers, the “hyomandibular,” in all its modifications, is treated of as part 
of the hyoid arch. In ancestral forms it may have been a distinct arch (see Anton Doury’s ‘ Studien,’ 
1885). 
