Distinctions.—This Isotelus resembles the gigas, from which, how- 
* _ ever, besides the aculeate processes, it is distinguished by the perfectly 
~ elliptic terminations, by the simple (not raised) margin of the shield, 
and by the proportions of the tail, the gigas having e length 4ths, 
and the megistos ths only of the width. The latter is also much more 
"prominent than the former, and the tail and sides much - more abrupt in 
their descent. From the megalops and the stegops it is clearly distin- 
guished by the eyes. 
_ History and mathematical proportions.—The first fragment (see out- 
line on Plate III) was discovered by myself in Adams county, Ohio, in 
1838. It was about six inches of the marginal “‘ cavetto” of the tail, 
beautifully veined, marked with the tubercle, perfectly elliptical, and 
coinciding with the énd of an ellipse twenty two inches long and twelve 
inches broad. The second specimen was an entire tail found at the 
same locality ; this, upon admeasurement, was found to coincide with 
an ellipse of exactly half of the dimensions of that which suited the 
first specimen, and showed, by a fortunate fracture, the internal mar- 
ginal cavetto. ‘These two specimens were both — and described 
by me in the Ohio geological report for 1839. 
The third specimen (see outline) was discovered in autumn of the same 
year by Wm. Burnett, Esq. on the hills at Cincinnati, and presented to 
me soon after. It was partly covered by the crystalline blue limestone 
in which it had been imbedded, and it was not until the winter of 1840- 
41 that I dissected it out of its gangue, and found that it hal an acu- 
leate shield, and that it exhibited the animal almost entire. 
It is of the same dimensions as the second specimen, and measures 
nine inches and three fourths in length, and six inches in breadth. The 
first fragment must therefore have been from a specimen nineteen inches 
and a half long, and twelve inches broad. These gigantic dimensions 
Suggested the name maximus, which I gave in the Ohio report, but which, 
for obvious reasons, I have changed to the more classical Greek term of 
the same import. 
_ The fourth specimen was. discovered by Mr. Carley, of Cincinnati, 
who was the first to discover the aculeate shield, for in the Burnett spe- 
cimen this character was still concealed. Mr. Carley’s specimen ap- 
pears to be a young one, for it is only about three inches long. It was 
obtained in the bed of the Ohio river about four or five hundred feet 
lower than the situation which furnished the Burnett specimen. My 
Own first specimens were found within thirty feet of the top of the blue 
limestone formation, where it is overlaid by the cliff limestone. Now 
_ the character of this magnificent species of trilobite has been ascer- 
tained, it is evident that fragments of it are abundant in our blue lime- 
stone, which is undoubtedly the equivalent of the limestone of Trenton 
*, 
