some British Fossil Crustacea. 405 



derately gibbous towards the base, but about one-half of the 

 front arched over to a vertical position (or at right angles to 

 the basal portion or plane of the body) ; axal furroivs consider- 

 ably less than half the length of the head, width of the in- 

 cluded space, or glabella, equal to two- thirds the length of the 

 head ; eyes small, near the lateral angles, their own length in 

 front of the posterior margin, two-thirds the width of the gla- 

 bella distant from the axal furrows. Length of head 10 lines, 

 width 1 inch 9 lines. 



This is only likely to be confounded with the /. crassicauda 

 (Dal.), from Gothland specimens of which it differs by the greater 

 width of the head and less depth of the deflected front, and most 

 remarkably by the very small size of the cheeks, resulting from 

 the eyes being removed almost to the lateral angles ; in the 

 /. ci'assicauda they are only half the width of the glabella distant 

 from the axal furrow, and the portion of the cheeks from the eye 

 to the lateral angles is nearly one- third more than from the eye 

 to the axal furrow, while in the present species the cheek beyond 

 the eye is little more than half the width of from thence to the 

 sides of the glabella. Heads of the Dysplanus centrotus (Dal.) sp. 

 differ in their much greater proportional length. 



In the Lower Silurian limestone of Wray quarry. Upper Tweed. 



{Col, University of Cambridge.) 



Isotelus affinis (M^Coy). 



Ref. Isot. gigaSj I. planus^ and /. Powisii of Portk. Geol. Rep. 

 (omit synonyms) t. 6. f. 1, and t. 9. f. 2 & 3. 



Sp. Char. Axis of the body only slightly exceeding the pleurse in 

 width ; pleurae gently arched downwards at a very obtuse angle 

 from about halfway between the axis and the extremity; a 

 large pleural furrow reaches from the axis to about one-third 

 of the truncated extremity of each ; pygidium flattened, semi- 

 elliptical, or slightly trigonal from the straightness of the sides ; 

 axis narow, sharply defined, gently convex, reaching as far as 

 the concave space round the margin. 



In general proportions this resembles the /so /e/ws^?^as(DeKay), 

 from all the varieties of which it is distinguished, when speci- 

 mens of the same size are compared, by the much greater flatness 

 or depression of all its parts, the long, narrow, sharply defined 

 axal lobe of the pygidium, and the much greater length of the 

 pleural groove of the pleurse (nearly double that of the /. gigas), 

 as well as the distance of the knee from the axis, and slight degree 

 of deflection of the pleurse (being bent nearly at right angles at 

 one-third from the axis in 7. gigas). The pygidium differs from 

 that of the /. Powisii (Murch. sp.) by the absence of all seg- 



