DR. A. GUNTHER’S DESCRIPTION OE CERATODUS. 
521 
transmutation of the cartilage. A layer of white connective tissue sometimes binds the 
bone and cartilage together ; hut generally the bone rests immediately upon the cartilage. 
(See, for instance, the vertical transverse section through the mandible, Plate XXXV. 
fig. 5, or through the vertebral segments, Plate XXXVIII. figs. 3-8.) Most of the 
bones are thin, sometimes flexible; and those which are thicker, like the ethmoid, 
mandibularies, and pterygo-palatine, have a spongy texture with numerous small 
medullary cavities (Plate XXXV. fig. 5). Bone-corpuscles are found in nearly all the 
ossified portions of the skeleton ; they have but few and very short processes. Their 
shape is subject to great variations; for instance, they are short in the sclero-parietal 
(Plate XXXIV. figs. 5 & 6) and other parts of the skull, and very elongate in the 
articulary hone of the mandible (Plate XXXV. fig. 6). 
General Configuration of the Skull. — The . Relations of its Cartilaginous and Osseous 
Parts (Plates XXXIV. & XXXV.). 
The skull consists of a completely closed inner cartilaginous capsule (Plate XXXIV. 
fig. 2, and Plate XXXV. fig. 2) and an outer incomplete osseous case, to which, again, 
some outer cartilaginous elements are appended. In the former the confluence of carti- 
lage is so complete that no distinct divisions are traceable by sutures ; its parts can be 
designated only by reference to the locally corresponding bones of the teleosteous skull. 
The bones of the outer case have their outlines more or less distinctly marked ; but, 
although they are few in number, their determination is very difficult. The vertebrate 
skull which approaches most closely the type of Ceratodus is that of Lepidosiren ; and 
nearly every part of this latter has been differently named by its describers. 
In order to facilitate the description of this skull, we may distinguish four regions of 
the cartilaginous capsule , viz. : — (1) the central region, much depressed, extending from 
the occiput to the orbit, enclosing the brain and auditory apparatus, raised in the middle 
into a slight longitudinal crest ; (2 & 3) a lateral region on each side of the former, in 
which the cartilage is expanded into a broad thin roof (Plate XXXIV. p) covering the 
gill-cavity, and which, anteriorly, is formed into the suspensory pedicle (s) ; (4) the facial 
region, a continuation of the central, but much more narrowed, tapering in front, and 
armed (in its vomerine portion) with two incisor teeth. 
The bones which form the outer osseous case , and will be described in detail hereafter, 
are attached to the cartilaginous case in the following manner. The upper side of the 
facial region is covered by a triangular bone ( ethmoid ) (Plate XXXIV. h , and Plate 
XXXV. h) ; it is intimately united with the underlying cartilage (which is perforated 
by the olfactory nerves), and forms the roof over the nasal cavity. At its hinder angle 
this bone forms a serrated suture with a second bone (os fronlale ) (Plate XXXIV. c, 
and Plate XXXV. c ), which emits a process downwards and in front of the orbit, to 
form a firm connexion with the palatine portion of the os pterygo-palatinum. This 
latter bone (Plate XXXIV. I, and Plate XXXV. /) occupies the edge of the lower side 
of the facial region, and bears the large upper pectinated tooth. It is narrow, but very 
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