48 GEOLOGY AND HINEEALOGY. 



Museum of the Geological Survey of Canada has lately received some very perfect 

 specimens of that species from the neighbourhood of Whitby ; and he kindly 

 promises us a drawing and revised description of his original species, for an ensuing 

 number of the Journal. The only figures of Asaphus latimarginatus that we 

 have had an opportunity of examining, consist merely of two more or less imperfect 

 caudal shields given in the first volume of Hall's Palteontology of New York. 

 Neither thorax nor buckler has, we believe, been hitherto figured or described — 

 at least beyond the brief description given in our note in the number of the 

 Canadian Journal already alluded to. If the two forms prove tQ be identical, the 

 original name of A. latimarginatus, as applied by Prof. Hall to the species 

 founded on the two imperfect caudal shields figured by him in his Palaeontology, 

 must, of course, take the place of A. Canadensis, notwithstanding the appropriate- 

 ness of the latter. Up to the present time, indeed, it is only in Canada that anything 

 like complete specimens have been met with. The following is a description of the 

 form to which our original remarks applied : 



Cephalic shield pointed anteriorly, and in its general outline closely resembling 

 that of Asaphus plat ycephalus* , but with the posterior angles terminating in hornsf 

 which extend downwards to the bottom of the fourth thoracic segment. Facial sutures 

 united in front at the extreme anterior margin of the buckler, and terminating as 

 in A. platycephalus about midway between the glabella and the angles of the 

 head-shield. Glabella very feebly raised ; broad, and somewhat squared above ; 

 but without furrows of any kind. Eyes apparently as in A. platycephalus, but 

 much destroyed in all the specimens examined. Eor dimensions, see below. 



Thorax with eight segments. Pleurae somewhat sabre-shaped (the curve up- 

 wards:):) ; grooved to about half their length from the axis outwards, and then 

 crossed obliquely by a curvilinear ridge : the points of the pleurae beyond the 

 ridge, delicately striated. 



Caudal shield with well developed axis : the axis tapering, and terminating 

 rather abruptly before reaching the extremity of the pygidium ; number of the 

 rings not observable in the specimens examined. § Pleura; 14 in number, without 

 grooves or ridges ; bent downwards abruptly near the striated margin into 

 which they merge. The lower ones, almost vertical. 



Whole surface of the trilobite finely punctured, except at the striated limb. 

 The punctures on the pleurae, larger and farther apart than those on the axis. Also 

 of a cre3cented or semi-circular form, with the convex and more deeply indented 

 side turned inwards.' 



Relative (approximate) dimensions : — Assumed length of Buckler =1. Glabella, 

 length =-812. Thorax, length =-8*75. Pygidium, length =1-06. Middle lobe 

 of Thorax, bread h ="50 to '60. Outer lobes (each) breadth =•'70. The small 

 breadth of the middle lobe in relation to the side lobes, as compared with Asa- 

 phus platycephalus, appears to be of some importance, unless it be a mere sexual 



• Isotelus giyas, Auct. 



+ This part of the head-shield is very obscure in the specimens hitherto examined. We 

 were led at first to believe that the angles were rounded. 



% This, however, is only to be seen when the pleura} have become accidentally separated 

 to a certain extent from one another. 



§ Since the above description was written, the son of His Excellency the Governor Gene- 

 ral, has kindly submitted to us some specimens obtained by him personally from the 

 Whitby quarries. In one of these, the pygidium of a young individual, fourteen rings may 

 be counted in the axis. 



