8 NOTES AND COMMENTS [january 



trilobite genus Cheirurus, or the carapace of the modern Zithodes arctica. 

 Among the terrestrial Isopoda, usually so smooth, Cubans aculeatus, 

 Budde Lund, breaks out into spikes all over its body. Sometimes the 

 soft abdomen of a spider becomes a regular fortress of obdurate angles. 

 The new Phyllocarid would look rather less formidable were its valves 

 closed, in what one may suppose to have been their normal position. 

 External pressure may account for their being spread so widely open. 

 A sign and result of this pressure is probably to be seen in the strong 

 diagonal groove which each valve in the fossil exhibits. 



From the same group Mr. Clarke is now able to re-describe another 

 important crustacean, if a trilobite be a crustacean. In 1888 he 

 founded the species Bronteus senescens on " a very imperfect fragment 

 of a pygidium." Now, he has had at command " two essentially entire 

 specimens." As so often with trilobites, the entirety refers only to the 

 dorsal view, but this enables Mr. Clarke to enter upon an interesting 

 discussion of the structural variations within the genus Bronteus, as 

 compared with the times of appearance of its several species. B. senes- 

 cens is said to be noteworthy, not alone for the rarity of all trilobites 

 at the horizon indicated, but as in all probability the latest representa- 

 tive of the genus. The specific name, therefore, must be understood 

 to refer rather to the genus than to the species itself as " growing old." 

 For the species is regarded by Mr. Clarke as " a survival, with appro- 

 priate time modifications, of the proper expression of the genus." 

 Evidently the representative of Bronteus was doing its best to march 

 with the times, but there must have been some revolutionary spirit 

 abroad ruthlessly bent on ending the dynasty of the Trilobites. The 

 Phyllocarids are still with us. 



More North American Fresh- Water Copepods. 



We have lately received two further instalments of the Bulletin of the 

 Illinois State Laboratory of Natural History (vol. v. 1898), a publica- 

 tion to which the attention of our readers has been called on several 

 previous occasions. The first (pp. 225-270) contains a paper by F. 

 W. Schacht upon the comparatively rare fresh-water Copepods belong- 

 ing to the genera Osphranticum, Limnocalanus, and Epischura. In a 

 notice of a previous paper by the same author on the closely allied 

 but much commoner genus Diaptomus (Natural Science, xii. p. 300), 

 it was pointed out that America possesses, so far as that genus is con- 

 cerned, a very characteristic fauna — not a single one of its species, for 

 instance, being known on this side of the Atlantic. It appears that 

 the same phenomenon is almost equally true of the remainder of the 

 forms included in the family Ccntropagidae, now under consideration, 

 for of the five American species dealt with, only one, Limnocalanus 

 macrurus, Sars, is known to occur outside North America. In this 

 case the animal is capable of living in both fresh and salt water, so 



