88 NEW YORK STATE MUSEUM 



the triliobite stock and differentiation therefrom at a very early 

 period of geologic history. 



The genus A g 1 a s p i s Hall, described as a trilobite from the 

 Cambric (upper) of Wisconsin, was, in Eastman's edition of 

 Zittel's Texthook of paleontology, removed by the writer from the 

 trilobites and placed under the family Aglaspidae of the order 

 Synxiphosura Paekard, in immediate association with the Hemi- 

 aspidae (1899, p. 672). Aglaspis eatoni Whitfield^ a figure 

 of which is here reproduced, has a lobed head-shield, low, conate 

 central area, approximate and small compound 

 eyes and no facial sutures. Its abdominal seg- 

 ments are flat and blade-like, markedly simi- 

 lar to those of Pseudoniscus, and the 

 body ends in a similar telson spine. Its seg- 

 ments have been reported as seven in number. 

 It clearly represents an early stage of the stock 

 to which the hemiaspids belong, nearer to the 



Aglaspis eat on Whitfield ,.,,., , ^ 



Upper Cambric, Lodi Wis. point of departure from the trilobite stock, 

 and yet it may be said that the slight difference in the expression 

 of the type after the long interval from Cambric to late Siluric 

 shows its stability. 



With the evidence afforded by these recent acquisitions of 

 Pseudoniscus we may submit the following diagnosis of 

 the genus: 



Pseudoniscus Nieszkowski 1859. Animal small. Head- 

 shield relatively large, convex; eyeless; no facial sutures; genal 

 angles extended into short spines; surface very obscurely marked 

 by radial furrows (impressions of appendages?). Abdomen strongly 

 trilobed and trilobitiform. Segments 10, flat, smooth; those of 

 the preabdomen 1-5 with o'bliquely grooved pleux^ae; sixth and 

 seventh segments partially conjoined on the pleurae; segments of 

 postabdomen narrow, lanceolate and with increasing retral cur- 

 vature. Telson a simple shor-t, straight spine. 



