324 Identity of Atops trilineatus with Triarthrus Beckii. 
of the entire fossil were first shown, though an errer was commit- | 
ted in describing two species, from forms now known to be 
pendent on the presence or absence of the maxillary portions of 
the buckler. This Trilobite appears upon further examination to 
be a Calymene, and the sesek attending its determination has 
n partly from the separation of the marginal portions of the 
cephalic shield, at the suture, leaving the central part as the usual 
representative of its form. This part of the fossil also presents 
a considerable variety of form dependent on the nature and con- 
dition of the enclosing matrix. [t occurs rarely in the Trenton 
limestone, but is abundant in the Utica slate, where the speci- 
mens are usually very much compressed and the parts separated. 
The crust.is extremely thin and the surface markings, under a 
magnifier, are similar to those of Calymene senaria, except being 
smaller and the points sharper. ‘The fine granulations or papille 
are well developed near the en of the central and lateral 
lobes, the more ex rts being (as in other trilobites) often 
smooth. A row of short spies or tubercles extend down the 
back on the central ae giving a prominent and unmistakable 
character to the species 
The form and ceed of parts are variable, depending upon 
the degree of pressure suffered, and the direction in which the 
orce was applied. This condition is better illustrated by fig- 
ures of actual specimens than by any description. 
In the specimens, fig. 1 a, b, from 
the Trenton limestone, the propor- 
tions are nearly similar, the whole 
individual being convex, above, and 
probably presenting nearly its natural 
d proportions. The cephalic 
shield is semi-elliptical rather than 
semi-circular, the width being much 
less than twice the length as stated 
by Prof. Haldeman. The medial lobe of the thorax is widest at 
the base of the buckler not in the lower part, where the oir 
tions are reversed. The lateral segments are impressed with 
distinct groove which becomes wider wwiete the extremities. 
In fig. la, the lateral portions of the cephalic shield are sepa- 
rated. In fig. 14, these parts are partially preser 
The specimen, fig. 2a, isa nearly entire individual from the 
Utica slate, much compressed. The buckler is imperfect in the 
absence of the maxillary shields, and is nearly twice as wide as 
long; the proportions of the middle and lateral lobes of the tho- 
1b 
* For the character: ae affinities of =e fossil, see Paleontology of New York, 
nol. i; pp. 237, 250, 25 1, and 252, with references to plates and figures of the 
same W 
