some British Fossil Crustacea. 4.05 
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) ; aval furrows 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 hkely to be confounded with the J. 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 
I. crassicauda 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. gigas, I. planus, and I. 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 pleure in 
width ; pleure 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 /sotelus gigas (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 pleurz (nearly double that of the I. gigas), 
as well as the distance of the knee from the axis, and slight degree 
of deflection of the pleure (being bent nearly at right angles at 
one-third from the axis in J. gigas). The pygidium differs from 
that of the J. Powisii (Murch. sp.) by the absence of all seg- 
