Miss Agnes Crane — On Certain Living and Fossil Fiskes. 209 
III. — On Certain Genera of Living Fishes and their Fossil 
Affinities . 1 
By Miss Agnes Crane. 
N first thoughts, it may see'm that the lowest group of verte- 
brates, of all the divisions comprised in the animal kingdom, 
might be niost easily described, and its zoological limits defined ; 
but, on examination, the fishes prove to be most curiously linked to 
the invertebrata below and tbe amphibian reptiles above. In fact, 
it is not easy to draw the lines positively between them, and to say 
where the true vertebrates begin, or where the piscine characters 
are merged in tbe reptilian. It is proposed to refer to some of 
the most aberrant forms of living fish and their fossil affinities ; 
then, briefly passing in review the distribution of the various 
families in geological time, to see how far descent with modification 
is traceable in this dass of vertebrates. 
The Lowest Vertebrate. — It is well known that the lowest ver- 
tebrate form is the anomalous lancelet ( Amphioxus lanceolatus) , which 
is found burrowing in sand banks on our Southern shores and in the 
Mediterranean. The position which this singulär species should occupy 
in the animal kingdom has long been a subject of debate among natu- 
ralists. Some, like Agassiz, separate it entirely from all other fishes, 
while Haeckel proposes to place it in a distinct division of the Verte- 
brata, and Professor Semper removes it from the vertebrates altogetlier. 
But Professors Owen 2 and Huxley , 3 considering it to possess the 
rudiments of a skull and brain, with the elements of a vertebral 
column, retain it among the fishes, and it forms the first or lowest 
Order of their respective svstematic arrangements. In Amphioxus, 
which ranges from. one and a half to two inches in length, the 
vertebral column is notochordal throughout life, that is to say, com- 
posed of a membraneous rod inclosed in cartilage; and as there is no 
enlargement of the skull for the reception of the brain, the animal 
tapers nearly equally at either end. The skin is scaleless, lubricous, 
and so transparent that the internal structure is visible, and the eyes 
are not more fully developed than in the common leech. The mouth 
is vertical, jawless, and suctorial, and is furnished with vibratile 
cilia. The lancelet possesses neither heart nor swimming bladder, 
and is without ribs and even rudimentary limbs. In all other fishes 
respiration is effected by means of water passing through the mouth 
and escaping by the gills, or their equivalents ; in this species 
it traverses the whole interior of the animal and escapes by a special 
pore on the under surface of the body. Professor Goodsir 4 long ago 
called attention to this peculiar mode of respiration, and noticed the 
1 Bead before the Brighton Xat. Hist. Soc. Feb. 8th, 1877, and the Geologists’ 
Association April 6th, 1877. 
The writer is much indebted to Dr. Günther, F.R.S., and Mr. "W. Davies, F.G.S., 
of the British Museum, for information kindly imparted, and facilities and assistance 
aiforded in the examination of specimens. 
2 Anatomy of Vertebrates, vol. i. 
3 Preliminarv Note on the Structure of the Skull and Brain in Amphioxus lanceo- 
latus, Proceedings of the Boyal Society, 1874, No. 157, December. 
4 Transactions of the Boyal Society of Edinburgh, vol. xv. pl. 11, p. 259. 
DECADE II. — VOL. IV. — NO. V. 14 
