Largest Australian Trilobitc hit/icrto discovered. 191 



the axis short and i-udinientary ; the coalesced pleural segments 

 flat, broad, and typically seven to eight on either side, with a 

 peculiar downward curve very characteristic of the genus. In 

 many species the limb is also wide and well-marked. 



Of the three families thus selected by elimination, the IlhBnida; 

 may, 1 think, be discarded, leaving only the Asaphidfe and 

 Bronteidse to choose from. 



Now, however obliquely distorted Trilobite pygidia may be, 

 take for instance the Asaphidaj of the Tremadoc Group, amongst 

 Lower Silurian forms, the axis is invariably perceptible to a 

 greater or less extent ; and, had there been such an axis on Mr. 

 Sweet's specimen, some trace of it would be visible, notwith- 

 standing the adherent matrix, more particularly towards the 

 apex. This, it seems to me, debars the entry of this fossil 

 amongst the Asaphidse ; although, it must be admitted, excepting 

 this character, and the absence of anterior lateral fulcral-facets, 

 the present fossil has a general resemblance to some of the Asaphi 

 proper, particulai-ly such species as Asaphus centralis., Conrad.* 



With regard to the Bronteidte, and a comparison with this 

 fossil, we ai^e met at the outset with the same axial difficulty. 

 The small lobiform axis is usually a prominent feature, and should 

 haA'e left some evidence of its presence, especially along the 

 anterior margin, although the specimen has certainly been 

 damaged here by blows from the hammer. There should likewise 

 have been traces of the long terminal appendage as a continuation 

 towards the apex of the pygidium, and the anterior fulcral-facets, 

 but both are conspicuous by their absence. The only remaining 

 feature on which to effect a comparison is that of the coalesced 

 pleural segments, and these are certainly more Bronteiform than 

 Asaphus-X'ik.Q. In Asaphus and Ogygia, the coalesced segments 

 are sometimes grooved and at other times not, but the angle that 

 each segment forms with the median axial line is an obtuse one, 

 at any rate in the anterior portion, and the whole radiate, as it 

 were, from the axis throughout its entire length. In Bro7iteus, 

 on the other hand, the similar angle is acute, the segments, in 

 consequence of their trend from the small axial lobe at the 

 anterior end of the pygidium, have a much greater backward 



* Wliitfieia, Bull, j^merican Mus. Nat. Hist., 1889, ii., No. 2, t.l2. 



