366 New Species of Trilohite. 



Art. XVIL — On a New Species of Trilohite of very large size ; by 

 John Locke, M. D., Prof, of Chem. and Pharm. in the Medical Col- 

 lege of Ohio.* Communicated to this Journal by the author. 



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 nearly 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 ; iniddle lobe cylindrical ; articulations eight, 

 bending flatly over 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 distinctly marked by 

 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- 

 fully marked with a " venalian" of eccentric curved and branched lines. 

 The above named posterior tubercle is very nearly the " focus" of the 

 " elliptic" outline of the tail, is just anterior to the marginal cavetto, 

 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 until they successively disappear on the anterior margin of the 

 " cavetto." 



* Read before the Association of American Geologists at Philadelphia, April 6, 

 1841. 



