366 New Species of Trilobite. 
Arr. XVIIL—On a New Species of Trilobite of very large size; by “a wa 
Joun Locxg, M. D., Prof. of Chem. and Pharm. in the Medical Col- is, 
lege of Ohio.* Communicated to this Journal by the author. ee 
Isotelus megistos. 
Clypeo, antice elliptico attenuate marginato postice arcuato, et ter- 
minato utrinque aculio ; cauda postice elliptica, antice arcuata ; articu- 
lus abdominis octo. 
The shield is anteriorly n pa perfectly elliptical, broadly and thinly 
margined, posteriorly arcuate, and terminated at the angles by spines 
or pointed processes extending backwards beyond the two first abdom- 
inal articulations. The eyes are prominent, large, furnished exteriorly 
each with a crescent-shaped cornea, and placed rather nearer to the 
posterior edge than to the outer margin of the shield. From the corner 
of each eye a sutural. line extends forward, meeting at. the anterior 
margin of the shield, and enclosing a lozenge-shaped, leaf-like frontal 
space. Abdomen trilobited; middle lobe cylindrical ; articulations eight, 
bending flatly ove# the middle lobe, and descending abruptly at their 
lateral extremities, which are broad, flat, and rounded beneath, and ad- 
mirably fitted to sliding over each other when the animal should con- 
tract or roll himself, according to a well known habit of the genus. 
Tail posteriorly elliptical, anteriorly circularly arcuate, length measur- 
ed horizontally, less than two thirds of the width, having two obscure 
longitudinal depressions continuous with the abdominal furrows, and - 
converging towards an obscure posterior tubercle. The anterior out- 
line of the tail exhibits three slight lobes, (corresponding with those of 
the abdomen,) the two, exterior of which are very uidad marked “4 
a transverse depression. 
When the posterior shell of the tail is decorticated an interior ‘shell 
is exposed, which forms all round a deep trough or “ cavetto,” beauti- 
i marked with a‘ venalian” of eccentric curved and branched lines. 
bove named posterior tubercle is very nearly the “ focus” of the 
« elipic” outline of the tail, is just anterior to the marginal cayetto, 
and is the centre around which the curved lines originate, each passing 
a little further back than the other and advancing outwardly and for- 
ward una, they. ania disappear on the anterior, margin of the 
“* cavetto. 
* Read before te ‘nialhainatibinsitetete Catia Philadelphia, April 6, 
1841, - Had a Be hte? s ; i nt! 
