234 cockerell: uropods of acanthotelson stimpsoni 



near it, the compensator may be used, not only to correct the 

 loss of light resulting from a lack of correspondence between 

 the planes of vibration of skylight and polarizer, but also to 

 adjust the light to the needs of the worker, either by an incom- 

 plete correction of the light loss or, where there is already es- 

 sentially complete correspondence between the vibration planes, 

 by a reduction of the illumination through a reverse movement of 

 the compensator. 



PALEONTOLOGY. — The uropods of Acanthotelson stimpsoni. 

 T. D. A. Cockerell, University of Colorado. 



Among some fossils kindly presented by Mr. L. E. Daniels to 

 the University of Colorado Museum is a specimen of Acanthotel- 

 son stimpsoni Meek & Worthen in a nodule from the Carbon- 

 iferous of Mazon Creek, Illinois. I have examined many ex- 

 amples of this species, but the present one is re- 

 markable for the perfect preservation of the uro- 

 podal rami, permitting a more exact interpretation 

 of their structure than was possible to Meek and 

 Worthen, or to Packard. 1 The rami are about 

 9.5 mm. long, hard and perfectly spiniform, and 

 strongly longitudinally grooved. The outer one is 

 straight, the inner gently curved. Stiff spinelike 

 bristles occur at intervals of somewhat less than a 

 millimeter on both sides of the inner ramus; and 

 also on the outer side of the outer ramus, where 

 they are more closely set. There are in addition 

 many very fine, soft setae fringing the rami, forming 

 an especially long fringe on the inner side of the 

 outer ramus. Packard's figure is, therefore, in er- 

 ror in showing numerous quite closely set bristles of one sort only. 

 Acanthotelson is an animal of more than ordinary interest. It 

 belongs to a group of Crustacea which Packard named Syncarida, 

 peculiar freshwater Malacostraca in which there is no carapace 

 whatever. In the Carboniferous and Permian strata of the 



Fig. 1. Uro- 

 podal rami of 

 Acanthotelson 

 stimpsoni. 

 About scale 4. 



> Mem. Nat. Acad. Sci., 3: 15th Memoir, pi. 1, fig. Id. 1886. 



