26 BRITISH CAMBRIAN TRILOBITES. 



elongated tubercle, which extends a quarter of the length of the glabella, and is 

 equidistant from its two ends; from the sides of this tubercle in front two narrow 

 grooves 1 curve forwards and outwards, approximately at right angles to one 

 another, as far as the axial furrows. 



" Thorax rather deep ; central division of the axis, in the posterior segment, 

 equal in width to the articulating bosses on either side, larger in the anterior 

 segment ; pleurae curved forwards, posterior edges semicircular, posterior longer 

 than the anterior pleurae. 



" Tail a little shorter than the head, which it closely resembles except in the 

 possession of spines and the different furrows on the axis ; lateral margins in some 

 specimens curved, in others straight, passing back into spines which curve inwards ; 

 posterior margin well rounded ; limb a little narrower than the axis, and 

 a little narrower behind than at the sides. Axis more than two thirds the length 

 of the shield, rounded behind, slightly narrowed in front of the centre, broadening 

 in front ; anterior segment represented by two lobes at the anterior corners ; 

 second segment extending forwards between these, almost to the anterior margin, 

 at the sides somewhat longer than the anterior segment, bearing a median 

 longitudinal tubercle which is highest near the back of the segment, whence it 

 extends forwards over the segment and backwards into the posterior segment, dying 

 down in each direction; posterior segment occupying three fifths the length of 

 the axis. 



" Dimensions of complete specimen collected by Mr. R. Rhodes for the 

 Geological Survey: Total length, 13'3 mm.; head, length 5*6 mm., breadth 6*3 

 mm. ; tail, length 5"2 mm., breadth G'2 mm. 



"Affinities. — A. callavei and A. dux, associated on the same horizon, closely 

 resemble one another in general shape and relief, the flatness of the margins of the 

 head- and tail-shields, and the details of the thoraces. They differ greatly in 

 the length and shape of their glabella and tail-axis and in the furrows over 

 their glabella, in the form of which A. callavei is perhaps unique. Their 

 great differences are remarkable in being combined with such close similarities. 

 The suggestion presents itself that they may possibly be male and female 

 of the same species, but the differences are perhaps too great for this to be 

 entertained. Nevertheless, the resemblances are such that both can best be com- 

 pared with the same forms, viz., with A. glabratus and A. lentiformis, Angelin, and 

 A. sidenbladhi, Linnrsn. A. callavei also shows a close resemblance to .1. vir var. 

 coiicinnus, Matthew, from the Paradoxides shales of the St. John Group, Newfound- 

 land, which latter differs chiefly in the greater length of the tail-axis as well as in 

 the glabella furrow. 



1 I cannol avoid a suspicion that these grooves may lie adventitious. They are approximately in 

 line with cracks upon the cheeks, and may possibly have been produced by pressure upon the tubercle. 

 Nevertheless, the} seem like actual furrows and not cracks, P.L. 



