273 
[Foevste. 
erable. A sharp and very distinct, but quite narrow, groove divides 
the glabella from the occipital segment. A deep groove separates 
the glabella from the anterior raised border of the head. The 
border is narrow but strongty defined, its margin rounding into the 
facial sutures, which anteriorly to the palpebral lobes pass close to 
the glabella, and along the palpebral lobes follow pretty closely the 
outline of the anterior half of the glabellar lobes. The length of 
the entire head is 4.8 mm. ;of the glabella alone 3.1 mm. ; the width 
of the glabella including its lobes is 3.1 mm. excluding these lobes, 
2.5 mm. A faint transverse furrow near the middle of the glabella 
may represent one of the normal glabellar furrows. At Cumberland 
Gap, Tennessee, a specimen occurs of the same size and propor- 
tions, having the same broad occipital segment and the moderately 
broad groove defining the anterior border from the glabella. Oc- 
curring in sandstone the transverse glabellar furrow just described 
is not seen, and the sharp furrow defining the postero-lateral lobes 
of the glabella seems to separate these entirely from the glabella. 
Proetus , Foerste, of the Denison Univ. Bull., Vol. ii, PI. 
viii, fig. 5, if not identical is at least a closely related type. The 
sharp groove defining the postero-lateral lobes, suddenly becomes 
obsolete anteriorly so as to leave the anterior edge of these lobes 
attached to the glabella ; posteriorly they also decrease somewhat in 
distinctness. The rounding of the anterior edge of the head into 
the facial sutures is very well shown indeed. The chief difference 
between this form and those previously described is in the character 
of the groove between the anterior border of the head and the gla- 
bella, which is considerably broader than in the specimens just de- 
scribed. Thus this groove which in the Anticosti and Tennessee 
specimens is scarcely one-third of a millimeter broad, in the Ohio 
specimens is about .6mm. wide. Our studies on other genera, how- 
ever, have led us to believe that the width of the groove defining 
the anterior border of the head of tribolites is perhaps their most 
variable feature, depending usually upon a slight difference in the 
inclination of the anterior border. 
Another feature which leads us to believe that these forms are 
alike is the fact that both the Anticosti and the Ohio specimens 
have the glabella and head regions ornamented by fine microscopic 
transverse close striae, which can be seen only when well preserved 
and with a good lens. The Ohio specimens are taken as the type of 
the species. 
PROCEEDINGS B. S. N. H. VOL. XXIV 18 DECEMBER, 1889. 
