cockerell: uropods of acanthotelson stimpsoni 235 



northern hemisphere there is evidence of the existence of several 

 genera. Formerly it was supposed that the group became extinct 

 in later Paleozoic times, but in 1893 a living representative 

 (Anaspides Thomson) was discovered in deep pools in the moun- 

 tains of Tasmania. Still more recently two other living gen- 

 era have been found: Par anaspides G. Smith in Tasmania and 

 Koonunga Sayce in the vicinity of Melbourne, Australia. As 

 early as the Mazon Creek Carboniferous, the appendages known 

 as uropods had become greatly modified from a strictly primi- 

 tive type. In Palaeocaris typus Meek & Worthen, of which an 

 example from Mazon Creek is before me, the rami are flattened 

 and expanded, approaching the form usual in Malacostraca, and 

 evidently used for swimming, i.e., for propulsion in water. In 

 Acanthotelson, on the other hand, the rami are of a very different 

 nature, slender and spinelike, apparently suited for executing 

 springing movements (comparable to those of the Collembola) 

 in the soft sand or mud at the bottom of the water. Thus two 

 types of modification were present ; one became nearly universal, 

 while the other, that of Acanthotelson, died out. Acanthotelson 

 must be regarded as the type of a distinct family, Acanthotel- 

 sonidae, and the groups of Syncarida may be tabulated thus : 



Rami of uropods spinelike Acanthotelsonidae. 



(Acanthotelson Meek & Worthen. Mazon Creek Carboniferous.) 



Rami of uropods flattened swimming organs 1- 



1. First thoracic somite quite distinct, though short. . . . Uronectidae. 2 

 (Permian of Europe; Carboniferous of Europe and America.) 



1. First thoracic somite fused with head, the point of junction more or 



or less indicated by a groove (living forms) 2. 



2. Eyes sessile Koonungidae (Koonunga.) 



2. Eyes stalked Anaspididae. 



No dorsal hump; mandible with three-jointed uniramous palpus. 



Anaspidinae (Anaspides). 

 With a dorsal hump; mandible with four-jointed biramous palpus. 



Paranaspidinae (Paranaspides) . 



2 Packard called this family Gampsonychidae, basing it on Gampsonyx Jordan 

 & V. Meyer. It appears, however, that this generic name was earlier used 

 for a genus of birds; so the next available name, Uronectes Bronn, has to be used. 

 This is the Permian form; the Carboniferous one is Palaeocaris Meek & Worthen 

 (Praeanaspides H. Woodward). Palaeocaris typus has a more or less distinct 

 dorsal hump, though Packard's figure does not show it. 



