DK. A. GUNTHER'S DESCRIPTION OF CERATODUS. 561 
zoologist will deny that there must have been a continuity of the Dipnoous type; and it 
is only a proof of the incompleteness of the palaeontological record, that we have to 
derive all our information regarding it from only three so very distant periods of its 
existence. 
The Dipnoi offer the most remarkable example of persistence of organization, not in 
Fishes only, but in Vertebrates. On a former occasion I have shown that numerous 
recent species of fishes have survived from the period of the geological changes which 
resulted in the separation of the Atlantic and Pacific by the Central- American isthmus. 
In Ceratodus we have now found a genus which, as far as evidence goes, persisted 
unchanged from the Mesozoic era ; and in the Sirenidce , a family the nearest ally of 
which lived in the Palaeozoic epochs. 
Perhaps future palaeontologists will be able to demonstrate as complete a series of 
transitional stages from the Fish to the Amphibian as that obtained by the study of 
the living and therefore more accessible forms of Haematocrya. Zoologists have had to 
abandon the attempt to separate the two classes by one or several absolute characters ; 
and it is only the concurrence of either decidedly ichthyic or amphibian characters by 
which they refer a creature to the one or the other class. However, this is a problem 
upon which Ceratodus has only a distant bearing ; and I am satisfied if I have succeeded 
in showing its relations to Lepidosiren, and the connexions of both genera with the Palm- 
ichthyic type. 
Explanation of the Plates. 
PLATE XXX. 
Fig. 1. Ceratodus miolepis, male, ^ the natural size. 
Fig. 2. Skeleton of Ceratodus forsteri, the natural size. 
In the side view of the skull, the facial cartilages are preserved ; their extent 
will be apparent from a comparison of this figure with fig. 1 of Plate XXXV. 
The three slits on the side of the snout, in front of the eye, are places over 
which the cartilage does not extend ; they are perfectly closed by the skin. 
Fig. 3. Termination of the vertebral column of another example. 
a. Entrance into labial cavity. 
b. The extent of the labial cavity is indicated by dotted lines. 
c. Cavity in the interior of the infraorbital cartilage. 
1. Ligamentum longitudinale. 
m. Antebrachial. 
n. Termination of the notochord. 
t. Last segments of the confluent neural and hsemal elements. 
4 h 2 
