94 Walcott — Appendages of the Trilohites. 



I would go still further and form a class of the Trilohita and 

 one of the Merostomata. 



Two general facts lead me to think that the modern crustacean 

 is descendant from the Phyllopod branch and the Trilobita 

 from a distinct branch.* 1st. The Trilobita branch ex- 

 hausted its initial vital energy in Paleozoic time and disap- 

 peared. 2nd. The Phyllopod branch developed slowly until after 

 the Trilobita passed its maximum and then began its great dif- 

 ferentiation that approaches culmination in recent times. 



When the trilobite and phyllopod diverged from their com- 

 mon ancestral crustacean the trilobite began at once to differen- 

 tiate and to use its initial vital energy in developing new species, 

 genera and families. Probably two thousand species and one 

 hundred or more genera are known from the Paleozoic strata. 

 With this great differentiation the initial vital energy was im- 

 paired and the Trilobita died out at the close of Paleozoic time. 



The Phyllopod branch continued with little variation until 

 after the trilobite passed its maximum, and then began to differ- 

 entiate until to-day its descendents form the class Crustacea, 

 that coiTcsponds to the class Trilobita in Paleozoic time. 

 Springing from a common crustacean base the two groups have 

 many features in common, and in carrying out of details of 

 structure in the limbs and gills many striking resemblances 

 occur. It does not impress me that trilobites were true 

 Entomostracans or Malacostracans ; they have certain character- 

 istics in common, but these are not necessarily the result of 

 lineal descent one from the other but are the result of descent 

 from a common ancestral crustacean type of pre-Cambrian time 

 that lived in the pelagic fauna in which all the earlier types of 

 life were probably developedf and from which, as time passed 

 on, additions must have been made to the paleoutologic record 

 of geologic time. The Phyllopods, Ostracods and Trilobita are 

 clearly differentiated in the lower Cambrian fauna. Bernard is 



*Tliis view is only confirmatory of ttie result of the profound study of the 

 Apodida- by Bernard (The Apodidie Nature Series, 1S92). 



tSee Brooks' beautiful memoir on Salpa, with its sugiijestive theory of the 

 origin of the bottom faunas of the ocean and the early geolouric fauniis. 

 The Genus Sulpa, INlemoirs from the Biological Laboratory of the Johns 

 Hopkins University, II, 1893, pp. 140-177. 



