Pisces. | LOWER PALAZOZOIC VERTEBRATA. 575 
SECT, X.—VERTEBRATA OF LOWER PALAOZOIC ROCKS (Cambrian and Silurian). 
4th Sub-kingdom. VERTEBRATA. 
Body symmetrical, supported by an internal skeleton, composed of a median, dorsal, spinal column, formed 
of numerous vertebree or joints, which protect a continuous spinal cord, from the side of which the nerves of 
sensation and voluntary motion are given off to supply the body; the anterior end of the spinal cord is dilated 
into a large mass or brain, inclosed in a skull or dilated modification of the four anterior vertebree. Mouth 
anterior, provided with two jaws acting vertically in the line of the body, the lower jaw below or behind the 
upper. Blood red, from rounded disks; one muscular heart below, or in front of, the cesophagus and spinal 
cord. Respiration by gills, or lungs, supplied with water or air through the mouth. Sexes separate. 
The greater number have two pairs of limbs for locomotion, but never more. The bones are vascular, and 
continue to grow and be absorbed during life by the action of internal vessels (unlike the hard parts of 
Invertebrata), developed from cartilaginous or fibrous internal tissue hardened by phosphate of lime, &c. 
This sub-kingdom contains four Classes: Ist, Pisces; 2nd, Reptilia; 3rd, Aves; 4th, Mammalia. 
Ist Class. PISCES. 
The fishes form the lowest class in general organization of the Vertebrata, and were probably the 
first created examples of that sub-kingdom. They live in water and breathe by gills. The blood is cold and 
with rather large disks; the heart normally of one auricle and one ventricle. The bones have less earthy 
matter than in the other classes, and sometimes none; the Purkinjinian cells are long and slender. The limbs, 
when they exist, are developed only in the form of paired fins; the skull is frequently an undivided cartilage-like 
box, and in several groups the spinal column remains during life a continuous mucous cord, undivided into ver- 
tebral centres or joints, resembling the foetal condition of these parts in the other groups; in many fishes, 
however, both head and spine are ossified and divided into all their elementary parts. It is not the lowest types, 
in general organization, which retain in the adult state this embryonic condition of the skeleton. 
The classification of Fishes by M. Agassiz, into Cycloid, Ctenoid, Ganoid and Placoid, is now found by 
anatomists to be so very imperfect that it is abandoned almost universally, and some modification, such as the 
following of Miiller’s arrangement, is usually adopted. Only the two highest groups in the scale are found 
in the Paleozoic Rocks. 
Tribes :—Ist, Leptocardii ; 2nd, Cyclostomata ; 8rd, Teleostea (= Cycloida + Ctenoida Ag.); 4th, Ganoida ; 
5th, Placoida; 6th, Syrenoidea. 
5th Order. PLAcorpa. 
Syn. = (Placoida Ag.—Cyclostomata) =(Chondroptérygien Cuv.—Accipenseride) = Elasmobranchii Mill. 
Endo-skeleton cartilage-like (mucous), the skull of one undivided box; bodies of the vertebrz large, 
processes small; exo-skeleton never scaly, often of osseous points, plates or spines. Teeth with large bases 
[easc. 11. ] 4 i 
