Page:Popular Science Monthly Volume 77.djvu/154

148 of the entire phylum. The trilobites from their morphological features have been generally regarded as entomostracous crustaceans with relationships on the one hand to the Phyllopoda and on the other to the Merostomata, while the coalescing of the caudal segments suggests also a relationship to the Isopoda.

Vertebrate paleontology has also furnished much to morphology.

The Fishes would be but imperfectly known in their wonderful variety but for the fossil types. The problematical group Agnatha found only in the Silurian and Devonian affords no certain evidence of a lower jaw or paired limbs, and in some of the genera of the Ostracoderma mimic in a curious way the contemporaneous euripterids, which has led some to erroneously ally them with the Merostomata. The dermal armor of most of these forms is also a striking morphological feature.

Woodward divides the fishes proper into Elasmobranchii, Holocephali, Dipnoi and Teleostomi, and considers that the common ancestors of all were Elasmobranchs. Numerous fossil forms among the Elasmobranchs and Dipnoids as well as the Crossopterygians which have been thought by many to bridge the gap between the Telelostomi and Dipnoi have added largely to our knowledge of the phylum.

The Batrachians which consist to-day largely of diminutive forms were represented in the later Paleozoic and early Mesozoic by the Stegocephalia which contain the giant labyrinthodonts with their highly complex infolding of the walls of the teeth.

The Reptilians which began their existence toward the close of the Paleozoic became so numerous and diversified during the Mesozoic that this division of geological time has been referred to as the age of reptiles. Several orders of Saurians containing many giant types flourished during this time, but became practically extinct before the close of the period. With the adaptation of some for walking on their hind legs, of others for swimming, and still others for flight we have developed a great variety of morphological features that would never have been suspected from a study of living forms.

The Birds which are recognized as possessing certain dinosaurian relationships and were doubtless derived from one of the reptilian orders are unknown prior to the Jurassic. The Mesozoic forms are generalized, the tail at first not being atrophied and the pelvis imperfectly developed as in later forms. The vertebra? also had not acquired their saddle-shaped articulation while teeth were present in the jaws of the adults. Such forms certainly add greatly to our knowledge of the morphology of this class.

The Mammals which began in the early Mesozoic were represented throughout the Cenozoic time by highly diversified forms, many of which have left no descendants. The gradual evolution of the