222 NEW SPECIES OF TRILOBITE. 



placed rather nearer to the posterior edge than to the outer mar- 

 gin of the shield. From the corner of each eye a sntm'al line 

 extends forward, meeting at the anterior margin of the shield, and 

 enclosing a lozenge-shaped, leaf-like frontal space. Abdomen 

 trilobed ; middle 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 admirably fitted to sliding over each other when the animal 

 should contract or roll himself, according to a well-known habit 

 of the genus. Tail posteriorly elliptical, anteriorly circularly ar- 

 cuate, length measured horizontally, less than two thuxls of the 

 width, having two obscure longitudinal depressions continuous 

 with the abdominal furrows, and converging towards an ob- 

 scure posterior tubercle. The anterior outline 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 around, a deep trough or " ca- 

 vetto," beautifully 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 forward until they suc- 

 cessively disappear on the anterior margin of the " cavetto." 



Distinctions. — This Isotelus resembles the g-igas, from which 

 however, besides the aculeate processes, it is distinguished by the 

 perfectly elliptic terminations, by the simple (not raised) mar- 

 gin of the shield, and by the proportions of the tail, the gigns 

 having the length four fifths, and the megistos three fourths 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 distinguished by 

 the eyes. 



History and mathematical proportions. — The first fragment 

 (see outline on Plate VI,) w^as discovered by myself in Adams 



