NO. 3 CAMBRIAN TRILOBITES 177 



Acrocephalites americanus differs from Acrocephalites insignis 

 (pi. 25, figs. I, la) in its longer frontal limb and rim, proportionally 

 broader fixed cheeks, and 17, instead of 21, thoracic segments. It is 

 a much larger form than the genotype A. stenometopus (pi. 24, 

 fig. i), and differs in its longer and flatter frontal hmb and less ele- 

 vated boss on the frontal limb. 



Acrocephalites ? aster (pi. 26, figs. 9, 9 a-c) has a frontal boss much 

 like that of ^. stenometopus. 



Comparing the thorax of those species in which it is preserved, 

 we have the following result : 



Acrocephalites americanus (pi. 24, fig. 3), 18 thoracic segments. 



Acrocephalites insignis (pi. 25, fig. la), 21 thoracic segments. 



Acrocephalites multisegmentus (pi. 24, fig. 5a), 25 thoracic seg- 

 ments. 



The three species all have very small pygidia, and the same type 

 of thoracic segment and pleural furrow, although the pleural lobe is 

 much narrower in A. multisegmentus. 



The surface of the test of A. multisegmentus is more evenly 

 granulated than that of ^. haynesi. 



Under previous conceptions of the genus most of the species now 

 referred to Acrocephalites and Alokistocarc would have been in- 

 cluded in Ptychoparia. 



ACROCEPHALITES AMERICANUS, new species 



Plate 24, figs. 2, 2a-b, 3, sa-b 



This species is represented by beautiful specimens of the cranidium 

 preserved as silicified casts of the test attached to siliceous nodules 

 that occur in argillaceous shales. With one exception the specimens 

 of the dorsal shield are from the shale; they are flattened by com- 

 pression, and the fine surface characters of the silicified specimens 

 are lost except for traces of the larger tubercles. 



Thorax with 27 transverse, narrow segments ; palpebral furrow 

 narrow, rounded, and continued well towards the outer end of the 

 segment. Pygidium small; axial lobe with three rings outlined by 

 transverse furrows. 



Surface marked by strong, scattered tubercles with a minutely 

 granular surface between. 



The largest cranidium has a length of 10 mm., and belonged to a 

 dorsal shield that was about 40 mm. in length. 



For comparison with other species, see notes under description 

 of genus. 



