Mr. Blackwall on undescribed Species of Araneide. 481 
vations in defect by an unknown constant quantity 2, the 
: ; a 
calculated refraction for the are a will be —~ -% and that 
See ; : - 
for the arc C, ple Hence the ¢rue refraction for the in- 
termediate arc 6 must equal the difference of these two quan 
tities, and the constant error of the instrument will be the 
excess of this proportion of the are c (or a) above the quantity 
previously calculated. 
The several values of the refraction in terms of the arc, and 
that of the error of the instrument, deduced from the applica- 
tion of the above formula to various combinations of the four 
classes of arcs, are subjoined. 
Ref. Error of 
er, 
Sector. 
I. By the difference of Band A... j1, —24/ 
ii. Cand A... 737 —23 
II], -——_—_————- DandA... , -26 
Iv, ———---——— DandB... zi5 —27. 
The true value of the mean refraction is most probably the 
average of the two first ratios, or ;1.,, and that of the in- 
strumental error 23}", or about 2" more than the quantity de- 
rived from actual measurement. (See Lond. and Edinb. Phil. 
Mag. for 1833, vol. ii. page 334.) 
Ilkley, April 27, 1836. Joun Nrxon. 
LXX XIII. Characters of some undescribedSpecies of Araneidz. 
By Joun Brackwatt, Esq., F.L.S., §c.* 
Tribe, Tuxsire_x, Latreille. 
Genus, Walckenaéria hi 
7 galls - 1nl. 
Walckenaéria fuscipes, 
EPHALOTHORAX oval, convex and glossy, with a slight indentation 
in the medial line cf the posterior region; the anterior part, which 
is prominent and acute, is compressed and deeply indented on the sides, 
and has also a slight longitudinal indentation above: in front it is divided 
into two segments by a transverse groove. Mandibles conical, armed with 
teeth on the inner surface, and inclined towards the pectus, which is broad 
and heart-shaped. Legs moderately robust; the anterior and posterior 
pairs, which are the longest, are equal in length, and the third pair is the 
shortest. ‘These parts, with the maxille and lip, are of a brown colour ; 
palpi brown, the fourth and fifth joints being much the darkest ; the fourth 
joint terminates in two apophyses ; one, which is large, depressed, and hairy 
externally, overlaps the base of the fifth joint; the other, which is small, 
projects on the inner side; the fifth joint is oval, convex and hairy ex- 
ternally, concave within, comprising the palpal organs; they are highly 
* Communicated by the Author. 
Third Series. Vol. 8. No. 49. June 1836. 3C 
