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section the tapering end of the former may be traced wedged 
into the basal portion of the skull. The skull is a completely 
closed cartilaginous capsule, nearly entirely covered with super- 
ficial bones, to which, again, some other cartilaginous elements 
are appended. In the former the confluence of cartilage is so 
complete that no distinct divisions are traceable by sutures. 
The tegumentary bones may be designated as, 1, ethmoid; 2, a 
pair of ossa frontalia ; 3, a pair of ossa pterygo-palatina, bearing 
the upper molars ; 4, a single sclero-parietal ; 5, an os basale ; 6, 
an os quadratum ; 7, operculum ; 8, suboperculum ; and 9, man- 
dible. Distinct maxillary and inter-maxillary elements are not 
developed, but replaced by facial cartilages which pass into the 
suborbital ring. 
On the whole the structure of the skeleton reminds us much 
of that of the Sturgeons, Chimcera , and Lepidosiren , and of all 
the modifications by which it differs from these allied fishes none 
is of greater interest than the peculiar structure of the paddles 
(fig. 2). The fore-paddle is joined to the scapular arch by an 
oblong cartilage (forearm) and by a broad basal cartilage 
(carpus). The central part, which we have found externally to 
be covered with scales, is supported by a jointed axis of cartilage 
extending from the carpus to the end of the paddle ; each joint 
bears a pair of three, two, or one-jointed branches. The skele- 
ton of the hind-paddles is formed on exactly the same plan. 
This singular structure is of interest from several points of view : 
1 . The analogy of this framework to that of the caudal portion 
of the vertebral column is obvious, Ceratodus being not only 
truly diphycercal as far as the termination of the body is con- 
cerned, but also with regard to the extremity of its paired 
fins. The many-jointed pectoral axis may be compared to the 
series of neural and haemal apophyses, both forming the base 
to a system of superadded processes (here two or three-jointed 
branches, there neural and interneural, haemal and inter- 
haemal spines), which are destined to serve as supports to 
the surrounding soft parts, and more especially the rays 
of the fin. Further, as the heterocercal tail of the stur- 
geon is justly considered to be a later development of the 
diphycercal form, so the pectoral fin of those fishes proves 
to be nothing but the heterocercal modification of the 
diphycercal Ceratodont paddle. 2. The singular filamentary 
limbs of Lepidosiren prove to be typically the same as the paddles 
of Ceratodus , but there they are reduced to the simple central 
cartilaginous axis, with the addition of only rudimentary 
rays in the African species. 3. Professor Huxley has already 
drawn attention to the affinity existing between the limbs of 
Lepidosiren and certain fossil Gfanoids, of which impressions 
f paddles with scaly centres have been preserved (fringed or 
