1910] 



The Ottawa Naturalist. 



131 



Pig. 1. Isotelus arenicola. Out- 

 line of free cheek and portion of cran- 

 idium from Britannia. One-third 

 natural size. 



Fig. 2. Lioteltis arenicola. Outline of the 

 specimen collected by Richardson at Deschenes. 

 One-third natural size. 



line, three-fourths as long as wide; border concave. On the 

 specimen described, which is a cast of the interior, there are two 

 rather distinct ribs on each of the pleural lobes. The axial lobe 

 is hardly distinguishable. 



Width of thorax at back of fourth segment, 135 mm., width 

 of axial lobe at same point, 45 mm. ; length of pygidium, 98 mm., 

 width, 130 mm. The specimen is only very slightly flattened. 

 The total length of this specimen appears to have been about ten 

 inches. Another specimen, described below, seems to have been 

 at least four inches longer. 



This species is more closely allied to Isotelus gigas than to 

 any other species, the pygidia of the tw^o forms being almost 

 alike. The axial lobe of the thorax of the species here described 

 is, however, much narrower than in either /. gigas or /. maximus. 

 I. gigas has no genal spines at maturity, and /. maximus has a 

 shorter and more nearly semicircular pygidium. /. harrisi has 

 a wider and shorter cephalon, and /. plat y mar ginaius has a much 

 wider concave border around the pygidium. 



A rather remarkable specimen of this same species was 

 found by W. C. King, Esq., on the shore of Lake Deschenes at 

 Britannia. This specimen is an impression of the lower surface 

 of the trilobite, and shows a longitudinal ridge corresponding to 

 the central furrow along the axis of the ventral side of the animal, 

 10 pairs of transverse furrows, and the impression of the hypo- 

 stoma first noticed by my friend, Mr. Narraway. The doublure 

 of the pygidium has also left a wide smooth impression, but in 

 the cephalic region the hypostoma is the only portion of which 

 there are any traces remaining. The specimen was found by Mr. 



