200 PALAEONTOLOGY 



(swimming) habit, the eyes are close to the margin 

 of the head, so that they could look outwards as well as 

 upwards, and in these there is never a tail-spine. We 

 owe these observations to Prof. Dollo of Brussels. 

 Nothing is known of the limbs of Dalmanites. 



3. Triarthrus becki (Fig. 57) is a small trilobite of 

 which very beautifully preserved specimens have been 

 found in the Ordovician Utica Shale of Rome, New York 

 State. In form this is much less tapering than the pre- 

 vious forms, and ends more bluntly behind. The semi- 

 circular head-shield has a much less distinct marginal rim, 

 the glabella is of almost uniform width, with lateral furrows 

 which are but very slightly inflected from the transverse 

 direction. In all these points the head shows much less 

 difference from the thorax than that of Calymene or 

 Dalmanites, so that Triarthrus is a more primitive form. 

 The cheeks are each little more than half the width 

 of the glabella ; a long and narrow eye is in the centre 

 of each ; the facial sutures are opisthoparian, but cut the 

 hind margin very close to the genal angle, running 

 obliquely inward and forward (with a bend at the eyes) 

 to cut the anterior margin separately at a distance apart 

 not very much less than the width of the glabella. 



The thorax consists of fourteen free somites, of which 

 the first eight are of almost uniform width, after which a 

 slight tapering takes place : each bears a median 

 tubercle, as also does the last (occipital) somite of the 

 head and the first of the pygidium. The pygidium is 

 very short (about two-fifths as long as broad), and consists 

 of six somites fused. 



