New Species of Trilohite. 367 



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 the length fths, 

 and the megistos fths 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 end 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 figured 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 had 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 trilohite 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 



