f 
1883.] Zoology. 207 
have, caused its reduction to a rudimentary organ—% Walter 
Fewkes, Cambridge, Mass. 
ON THE ORIGIN.OF THE SPECIES OF Ocypopa, OR SAND-CRABS, 
FROM THE BONIN ISLANDS.—The species on which the following 
observations are made was obtained from the Bonin islands in the 
year 1880, It seems to be closely allied to Ocypoda arenaria 
Latr., described and figured by M. Edwards, in the Hist. Crust., 
Vol. u, Pl. xix, Figs. 13 and 14, 
mong numbers of specimens obtained, we can at once dis- 
tinguish two widely different forms; the one with the eye-stalk 
enormously produced beyond the edge of the carapace, and the 
other with the normal eye-stalk. The former has the anterior 
border of the carapace wavy ; the wave beginning with its high- 
est point on the exterior angle of the orbit descends gradually 
outwards till it ends, with its lowest point, on the external angle, 
which is about g0°. The lateral edge forms a concave face ex- 
ternally for a short distance. The anterior gastric groove has its 
concave face internally : 
In the short-eyed form on the contrary, the same anterior bor- 
der of the carapace is deeply concave anteriorly, the two forming 
an acute angle, which censequently points anteriorly. The anterior 
gastric groove has its convex face externally. Again a deep 
notch on the border of the orbit below, and near to the external 
angle of the carapace is found to be entirely wanting in the 
former, Lastly the .st abdominal somite but one is narrower in 
the former, and in the male, widened near the middle, but little 
posteriorly. The measurement of the whole abdominal somite 
shows similar proportions. 
I will here annex the measurement of the somite from a female 
crab, in which the eye measures 11™™, and the stalk beyond 
‘hes 8™" side by side with the normal or the short-eyed 
m. ’ 
Long-eyed. Short-eyed. 
20. t ea 
The greatest width of the somite, last but one...... katis pors 
t o De Teast widh ol the same... 0: cs O T 8.5 “ 9-5 * 
The greatest konb do OE ee er 11.0 “ 10.0 “ 
PNM do. E 9.0 8.1 
a following is the measurement taken from the entire 
mite : 
: : Long-eyed. Short-eyed. 
| ie yg a Me ya lice Pe i me 
scion eee eee eS 
the male, 
ale milar differences of measurement occur also in 
r need not cite here. ah hee 
- ra o great differences are, I think, enough to distinguish t ae 
a forms as two distinct species, had there not been a series o 
-OMS standing as stepping stones between them. F : 
the Comparison of a number of specimens we detect a form 
