. Chordate morphology. Morphology (Animals); Chordata. A B Figure 8-65. Section through tooth and jaw of alligator, A, showing second and third generation teeth in relation to functional tooth. B, vertical section through fully formed tooth. by Kiikenthal, Rose, and others, supposes that even the tri- conid tooth is a product of fusion of three simple teeth. In view of the compound nature of the placoid scale of Sqttalus, this seems most probable and would mean that all mam- malian and most tetrapod teeth are in fact polymers. The teeth of mammals, at least the molariform teeth, have an intric
. Chordate morphology. Morphology (Animals); Chordata. A B Figure 8-65. Section through tooth and jaw of alligator, A, showing second and third generation teeth in relation to functional tooth. B, vertical section through fully formed tooth. by Kiikenthal, Rose, and others, supposes that even the tri- conid tooth is a product of fusion of three simple teeth. In view of the compound nature of the placoid scale of Sqttalus, this seems most probable and would mean that all mam- malian and most tetrapod teeth are in fact polymers. The teeth of mammals, at least the molariform teeth, have an intricate pattern of cusps on them. This pattern, which involves a complex terminology, is useful in the tax- onomy of the group. The heterodonti of the mammal makes possible the use of a dental formula (Chapter 3). This de- vice indicates the numbers of the various types of teeth in the upper and lower jaws. Such a formula can have a tax- onomic value since it reflects an aspect of adaptive modi- fication. RESUME In terms of the skin and its derivatives, there are a few salient points. The stratified epidermis of the vertebrate sep- arates this group from the protochordates and invertebrates in general. The histology of the skin has not been used in subdividing the vertebrates. It can be noted that the epider- mis of the cyclostomes is thicker in terms of the number of cell layers than that of gnathostome and that the dermis is perhaps less complex in its structure. These differences can- not be sharply defined, however, since most of the agnaths are extinct and the structure of their skin is unknown. Among the gnathostomes, the amphibians are distinct in terms of their saccular or simple, alveolar, mucous and poison glands. The mammals in contrast have tubular, very coiled sweat glands. The general picture is much as before with three lines of tetrapods (amphibian, reptile, and mammal) stemming from some unknown ancestor. Neither the amphi- bian, with its specialized glands, nor th
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