250 C. D. Walcoft — Appendages of Trihhites. 



From the palaeontological record I am essentially in accord with 

 this view, but I am not yet prepared to abandon the position taken 

 in 1881, that all these groups should be arranged under one class 

 and not as an appendage to the Crustacea, as proposed by Dr. Lang. 



I would go still further, and form a class of the Trilobita 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.-' Ist. The Trilobite branch exhausted its initial vital 

 energy in Palgeozoic time and disappeared. 2nd. The Phyllopod 

 branch developed slowly until after the Trilobita passed its maximum, 

 and then began its great differentiation that approaches culmination 

 in recent times. 



When the Trilobite and Phyllopod diverged from their common 

 ancestral Crustacean the Trilobite began at once to differentiate 

 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 stiata. With this great 

 differentiation the initial vital energy was impaired, 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 differentiate, 

 until to-day its descendants form the class Crustacea, that corre- 

 sponds to the class Trilobita in Palaeozoic 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 occui". It does not impress 

 me that Trilobites were true Entomostracans or Malacostracans ; 

 they have certain characteristics 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 developed ^ and from which, as 

 time passed on, additions must have been made to the paleeontologic 

 record of geologic time. The Phyllopoda, Ostracoda, and Trilobita 

 are clearly differentiated in the Lower Cambrian fauna. Bernard is 

 confident that the Trilobites may take a firm place at the root of 

 the Crustacean system, with the existing Apus as their nearest ally.^ 



There is much yet to be learned from the study of Triartlirns 

 A great amount of material can be readily collected at the locality 

 near Eome, N.Y. It is also of interest to note that the locality at 

 Trenton Falls, N.Y., from which the specimens of Calymene and 

 Ceraurus were obtained, is only seventeen miles from the Rome 



^ This view is only coniirmatory of the result of the profound study of the 

 Apodidie by Bernard. (The ApodidtB ; Xature Series, 1892.) 



^ See Brooks' beautiful Memoir on Salpa, with its suggestive theory of the 

 origin of the bottom faunas of the ocean and the early geologic faunas. The Genus 

 Salpa, Memoirs from the Biological Laboratory of the Johns Hopkins University, 

 ii. 1893, pp. 140-177. 



3 "Nature," 1893, vol. xlviii. p. 582. 



