160 PROFESSOR OWEN ON THE DEVELOPMENT AND HOMOLOGIES OF THE 
median line anterior to the umbilical aperture {u), and arching' outwards, forwards, 
and slightly upwards to near the ends of the third pair of ribs : the hyposternals {ps) 
are represented by a similar transverse pair of slender cartilages, with a tendency to 
bifurcate at their extremities. The cartilaginous foundations of the episternals and 
xiphisternals have a not very definite linear form : the coracoids (fig. 1 a, 52 ) are more 
plainly distinguishable ; I at first mistook them for the episternals. The rudimental 
hyosternals and hyposternals at this period repeat the characters of the sternal or 
abdominal ribs (haemapophyses) in the Crocodile ; the entosternum represents the 
thoracic sternum of the Crocodile. 
The thick and somewhat dense corium of the carapace, covering the rudiments of 
the neural arches and pleurapophyses, when examined under a power of 300 linear 
diameters, does not present exclusively the fine filamentous interlaced structure of 
cellular tissue in progress of condensation into derm, as in the embryo bird ; but 
includes oblong nucleated cells, likeThose of cartilage, which along the middle line 
of the back are arranged in groups of linear series radiating from a centre corre- 
sponding with the point of convergence of each pair of neurapophyses, and connected 
with the extremities of those cartilages by a mass of cartilage-corpuscles holding the 
place of the neural spines. 
The cartilage-corpuscles in the firm semiopake part of the corium covering the 
ribs, show traces of linear arrangement at right angles to the ribs, or in the axis of 
the carapace ; especially near the proximal ends of the middle ribs. The thickened 
border which defines the carapace is formed almost entirely of oblong nucleated car- 
tilage-corpuscles, pretty closely aggregated and without observable definite arrange- 
ment. The stratum of cartilage-corpuscles in the substance of the corium of the 
plastron is thinner than that of the carapace : something like a linear radiated 
arrangement of these maybe discerned at the parts corresponding to near the mesial 
ends of the hyosternals and hyposternals ; but they are for the most part irregularly 
and more thinly scattered in the fibrous tissue than on the carapace. 
Homologically I conceive that this basis for future ossification, being situated in 
the substance of the skin, must be held to be the groundwork of a dermal skeleton ; 
and that, whether ossification extends into such basis from the subjacent ossifying 
parts of the endo-skeleton, or whether it commences independently in the dermal 
cartilage, and afterwards unites with the deeper-seated bones, does not affect such 
homological relation : in other words, that a dermal bony scute, whether it be con- 
nate or become confluent* with a part of the endo-skeleton, is still essentially a der- 
mal bone. 
But although, with regard to most of the superadditions to the endo-skeletal basis 
of the carapace, I have not been able to distinguish a period of the development of 
an independent centre of ossification, yet the superadded parts, ossified from pre- 
* I use the terms ‘ connate ’ and ‘ confluent ’ in the sense defined in my work on the ‘Archetype of the Ver- 
tebrate Skeleton,’ 8vo, p. 49. 
