72 
AUG. F. FOERSTE 
pleural segments of the thorax and permitted the free cheeks to 
separate frum the cranidium. Very few specimens that re- 
mained in their natural position escaped dismemberment during 
decay. Dr. Welch, now dead, but formerly also a resident of 
Wilmington, Ohio, found one of the few specimens ever seen 
imbedded in a natural position in the rocks, but the parts of his 
specimen were badly disarranged. These facts suggest a rapid 
deposition of the clay layers in which the large specimens of 
Isotelus were found. 
In Cincinnatian areas thin layers of limestone, several inches 
thick, frequently are interbedded with somewhat thicker layers 
of clay, often a foot or more in thickness. The limestone layers 
often consist of more or less comminuted fragments of bryozoans, 
shells, and other organic remains, and are remarkably free of clay 
except in very moderate quantities. Apparently the waters that 
stirred up the organic fragments and removed the clays from the 
future limestone layers kept these clays more or less in suspen- 
sion and later permitted their deposition when the violence of the 
motion of these waters had considerably diminished. In this 
manner considerable quantities of clay may have been deposited 
in relatively brief periods of time. 
Little is known of sex differences among the trilobites. The 
presence of both broad and narrow forms of Isotelus in the Rich- 
mond strata of Ohio and Indiana suggests the possibility that 
the more elongate forms {Isotelus maximus) may be the males, 
and the broader forms {Isotelus hrachycephalus) the females of 
the same species. Our present knowledge does not permit us to 
determine with any confidence whether the differences noted are 
connected with sexual differences or not. 
ACROLICHAS 
Ohio Journal of Science, XIX, 1919, p. 402. 
For the American species at present referred to the European 
genus Amphilichas, the generic term Acrolichas is proposed, on 
account of differences in the structure of the pygidia belonging 
to the American species. These pygidia have three pairs of ribs, 
