These nine classes of Vertebrate animals are, however, by no means of the same genealogical value. Hence we must divide them, as I have already shown in the Systematic Survey on p. 133, into four distinct main-classes or tribes. In the first place, the three highest classes, Mammals, Birds, and Reptiles, may be comprised as a natural main-class under the name of _Amnion animals_ (Amnionata). The _Amnion-less animals_ (Anamnionata), naturally opposed to them as a second main-class, include the four classes of Batrachians, Sea-dragons, Mud-fish, and Fishes. The seven classes just named, the Anamnionata as well as the Amnionata, agree among one another in numerous characteristics, which distinguish them from the two lowest classes (the single-nostriled and tubular-hearted animals). Hence we may unite them in the natural main group of _Double-nostriled_ animals (Amphirrhina). Finally, these Amphirrhina on the whole are much more closely related to those animals with round mouths or single nostrils than to the skull-less or tube-hearted animals. We may, therefore, with full justice class the single and double-nostriled animals into one principal main group, and contrast them as _animals with skulls_ (Craniota), or _bulbular hearts_ (Pachycardia), to the one class of _skull-less animals_, or animals with _tubular hearts_. This classification of the Vertebrate animals proposed by me renders it possible to obtain a clear survey of the nine classes in their most important genealogical relations. The systematic relationship of these groups to one another may be briefly expressed by the following table. A. =Skull-less Animals= 1. Tubular hearts 1. Leptocardia (+Acrania+) { _a._ Single nostriled { animals { 2. Round-mouths 2. Cyclostoma B. { _Monorrhina_ { =Animals with= { =Skulls= { b. Double { { 3. Fish 3. Pisces (+Craniota+) { nostriled { I. Non-Amnionate { 4. Mud-fish 4. Dipneusta { animals { Anamnia { 5. Sea-dragons 5. Halisauria or { { { 6. Batrachians 6. Amphibia { _Amphirrhina_ { =Thick Hearts= { { II. Amnionate. { 7. Reptiles 7. Reptilia (+Pachycardia+) { { Amniota { 8. Birds 8. Aves { { { 9. Mammals 9. Mammalia