148 TEE POPULAR SCIENCE MONTHLY 



pretation of the entire phylum. The trilobites from their morpholog- 

 ical features have been generally regarded as entomostracous crusta- 

 ceans 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 Ostra- 

 coderma 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 morpholog- 

 ical feature. 



Woodward divides the fishes proper into Elasmobranchii, Holo- 

 cephali, Dipnoi and Teleostomi, and considers that the common an- 

 cestors 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 Batracliians which consist to-day largely of diminutive forms 

 were represented in the later Paleozoic and early Mesozoic by the Stego- 

 cephalia 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 general- 

 ized, 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 mam- 



