176 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 64 



Acrocephalites ? majus (pi. 26, fig. i) is doubtfully referred to 

 Acrocephalites. It is slightly distorted, but there appears to have 

 been a boss in front of the glabella that was crossed transversely by 

 the narrow ridge that serves to separate the frontal limb and rim. 



Comparison of genera. — Acrocephalites differs from Alokistocare 

 (pi. 25) in having the boss in front of the glabella limited to the 

 frontal limb, and in having a more or less tuberculated outer surface 

 of the dorsal shield. The outer surface of Alokistocare is more or 

 less punctate in the type species, A. sitbcoronatum (pi. 25, fig. 2). 

 The typical species of each genus are readily distinguished, but 

 there are such species as Alokistocare lahrosum (pi. 25, figs. 5, 5a), 

 which have the Acrocephalites form of cranidium with the punctate 

 test surface of Alokistocare. 



Comparing the cranidium of Acrocephalites with that of Cteno- 

 cephalus exsulans Linnarsson, we find that both have a tuberculated 

 outer surface, rounded boss before the glabella within the frontal 

 limb and similar form of glabella. The absence of free cheeks and 

 eyes on the dorsal surface of Ctenocephalus is the essential difference 

 between the two genera. In Ctenocephalus the advance of the facial 

 suture and eye from the ventral to the dorsal surface appears to 

 have been retarded during the entire development and growth of the 

 cephalon. 



Some of the forms referred to Inouyia^ have a rounded boss on 

 the frontal limb, but they differ so much in the appearance of the 

 cranidium as a whole that, with the possible exception of Inouyia 

 titiana (Walcott), there is little risk of confusing them with species 

 of Acrocephalites. 



Comparison of species. — The cranidium of Acrocephalites steno- 

 metopus (pi. 24, fig. i) is nearer in form to that of A. haynesi (pi. 

 24, figs. 4, 4a) than to other species of the genus. It may have an 

 occipital spine, but of this we have no positive information. Acro- 

 cephalites tutus (pi. 24, figs. 6, 6a) is also to be compared with the 

 two mentioned species, but as the specimens representing it are 

 flattened in the shale, the element of convexity must be restored when 

 comparisons are made. Acrocephalites insignis (pi. 25, figs, i, la) 

 has the narrow fixed cheeks and frontal limb of Acrocephalites steno- 

 metopus, but owing to the compressed and more or less macerated 

 condition of the test it is not possible to draw detailed comparisons. 



Research in China, Carnegie Institution of Washington, Vol. Ill, 1913, pi. 14- 



