ISOTELUS, ACROLICHAS, CALYMENE, AND ENCRINURUS 71 
Considering how abundant the fragments of large specimens 
of Isotelus are at various horizons in the Richmond formation it 
seems strange that .no attempt has been made to illustrate them 
fully. Our knowledge of Isotelus maximus is confined to the few 
notes and the meager illustrations presented by Locke in his 
original publication. The numerous citations of Isotelus maxi- 
mus from other authors concern chiefly closely similar, but prob- 
ably not identical, Trenton forms. The Cincinnati species 
described by Locke under Isotelus megistos also needs further 
elucidation. It is hoped that the present publication of illus- 
trations of Isotelus hrachycephalus, and the accompanying obser- 
vations on Isotelus maximus and Isotelus megistos will stimulate 
an interest in the large specimens of Isotelus occurring in Ordo- 
vician strata. 
Accompanying his original description of Isotelus maximus j 
Locke pubhshed a figure of this species as though of an indi- 
vidual 21 inches in length. His figure was based upon a py- 
gidium of only half the size of that included in the figure, enlarged 
so as to correspond in size to a second pygidium of which he had 
only a fragment of the doublure. In a similar manner Clarke 
published a figure of Isotelus maximus, from the Prosser lime- 
stone at Mentorville, Minnesota (Geol. Minnesota, 3, pt. 2, 
1894, p. 706, plate) as though of an individual 17 inches in length. 
His figure was based on a fragment of a large glabella. Both 
figures are based on estimates; both undoubtedly represented 
large specimens of Isotelus; but actual figures and measurements 
of large complete specimens are preferable, and an accumulation 
of such figures and measurements is necessary before the large 
forms of Isotelus can be discriminated successfully into species. 
Both of the large specimens of Isotelus hrachycephalus, here 
figured and described, were found lying on their back imbedded 
in the middle of an indmated clay layer. In fact, all of the large 
specimens of Isotelus found by Dr. George M. Austin in place in 
the Richmond strata of Clinton and neighboring counties in 
southwestern Ohio, between ten and fifteen in number, were 
found imbedded in clay and lying on their back. Evidently the 
specimens were covered by clay before decay dismembered the 
