530 MULLER ON THE STRUCTURE OF THE GANOIDS, 
myzon and Myzxinoidei, are combined, whilst the great section 
of osseous fishes includes those which have proportionately but 
slight differences. 
Pallas and Agassiz have, it is true, separated a portion of these 
fishes, the Sturgeons, from the others. The former (Zoograph. 
Ross. Asiat.) placed the Sturgeons amongst the fish having 
opercula and free gills, which he called Branchiata, and opposed 
to them the order Spiraculati, which includes the remaining 
cartilaginous fishes, our present Plagiostomi, Chimere and 
Cyclostomi. Agassiz, who divides the fishes into four orders, 
Ctenoidei, Cycloidei, Ganoidei and Placoidei, very properly re- 
ferred the Sturgeons to the Ganoids ; and in like manner he had 
the Rays, Sharks, Chimere and Cyclostomi remaining, so that 
his Placoids are the same as the Spiraculati of Pallas. Although 
the Cycloids and Ctenoids cannot be continued as orders, still 
this division contains new and important elements in the deve- 
lopment of the natural system. The Ganoids, in a modified 
form, remain as a distinct order, and yield a portion of what has 
hitherto formed their contents to the vertebrate fish. But the 
Spiraculati of Pallas, the Placoidei of Agassiz, still combine the 
most perfect and the most imperfect fishes, which present the 
greatest differences in their anatomy. 
The Plagiostomi or Selachii of Aristotle, i. e. the Sharks and 
Rays, form an order of fishes which are quite peculiar through- 
out their entire organization, and differing from all others in 
their skull, which is undivided, but furnished with jaws; and 
in the covering of all the cartilages with the above-mentioned 
characteristic fine mosaic of tessellated pieces of bone, which 
does not recur throughout the whole system of fishes ; in its ad- 
herent gills having spiracula in the gill-cavities ; in the presence 
of the gill-arches ; the absence of the operculum in the generative 
organs ; for the young males have the peculiar external organs 
with the accessory testes, whilst in the females the tubes unite 
above the liver to form a single abdominal orifice; they also 
have the peculiar glands of the oviduct. The only fishes which 
are related to them are the Chimere, which possess a different 
kind of a finer osseous shell of the cartilages, identity of the 
intestines, similar structure of the external and internal male 
organs of generation, accessory testes, the external appendages, 
the ducts of the ovary, and even the same structure of the tegu- 
ment of the ovum. 
