144 CRUSTACEA. 



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. hot. gigasy I. planus, and /. Powisii of Portk. Geol. Rep. 

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



Sp. Cha?\ Axis of the body only slightly exceeding the pleurae in 

 width ; pleurcB 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 /5o/e/w5^2^fl5(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 /. gigas). The pygidium differs from 

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

 mental furrows, except the first, on the lateral lobes, and by the 

 more pointed outline and narrow margin. 



Not uncommon in a Lower Silurian schist over the iron-works 

 at Tremadoc ; very similar in appearance to that at Pomeroy, co. 

 Tyrone, which afforded the species to Col. Portlock. 



{Col. University of Cambridge.) 



Griffithides meso-tuherculatus (M'Coy). 



Sp. Char. Cephalothorax 10 lines wide ; glabella widely pyri- 

 form, broadly rounded in front, gently convex and narrowing 

 posteriorly with concave sides, very minutely granulated, length 

 5 lines, width 4 lines ; cheeks triangular, flat, smooth ; eyes 

 large, reniform, very minutely reticulated, with a large convex 

 eye-lobe * connected with the base of the glabella by a small, 

 oblique, oval nucleus ; limh broad, convex, with nine or ten 



* Eye-lobe seems preferable to eye-lid for the lobe covering the inner and 

 upper aspect of the eye. 



