SIXTH REPORT OF THE DIRECTOR IOXX) 43 



significance. In these palingenetic characters the nepionic stage 

 resembles the Cambric eurypterid Strabops (Strabops stage). 



The relationship to Limulus is indicated by a number of 

 larval characters common to both the eurypterids and Limulus. 



The prototype of the eurypterids has been reconstructed from 

 the palingenetic characters of the nepionic specimens and the 

 Cambric Strabops. By comparison of the later genera with this 

 prototype the conclusion is reached that two separate families 

 have been developed, the Eurypteridae and the Pterygotidae. 

 In the latter family the genus Hughmilleria evinces the most 

 primitive structure and Pterygotus and Slimonia have developed 

 in different directions. The main stem of the stock in the family 

 Eurypteridae is found by the g'enus Eurypterus which appears 

 as the earliest in the Clinton -and persists into the Permic. From 

 it branch off Eusarcus and the subgenus Onychopterus which 

 points the way to Dolichopterus, Drepanopterus and through 

 the latter to Stylonurus. 



The observations of the larval stages have been used in the 

 investigation of the taxonomic relations of the Eurypterida. It 

 has been found that the larval stages of the eurypterids and 

 Limulus have all important characters in common, and that the 

 differential characters are due to purely adaptive changes or 

 are cenogenetic. While thus the close relationship of the two 

 is also supported by ontogenetic facts, evidence is brought for- 

 ward for the conclusion of a Precambric separation of the 

 Xiphosurans and eurypterids. The larval stages of the euryp- 

 terids are further compared with those of the scorpions and 

 evidence of relationship found through descent from a common 

 ancestor. A comparison of the larvae of all three, the euryp- 

 terids, Limulus and the scorpion, has shown that both the latter 

 have lost the primitive form of the abdomen by acceleration in 

 which the eurypterids have best preserved the original gradual 

 and uniform contraction, and that all three are derivable from a 

 common ancestor to which the eurypterids are still nearest in 

 their general aspect. It is further inferred that this common 

 ancestor is more primitive than the crustaceans and may have 

 to be sought among the annelids. 



Monograph of the Devonic Crinoidea. Opportunity for the 

 prosecution of this undertaking is limited to the few weeks each 

 year that can be controlled by Mr Kirk who has been carrying on 

 the study for several years under the conditions indicated. The 



