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backwards): A double row of four pairs of spines extends back 
through the medial section of the dorsum as far as back edge of 
coxa three. Beginning over the front edge of coxa two, a sub- 
marginal row of three, and a marginal row of three, extend back 
as far as back edge of coxa three. The longest pair of spines in the 
dorsum are situated behind and equally distant from the terminal 
spines of the medial and sub-marginal rows of their respective sides. 
Around the posterior end of the abdomen on dorsal and ventral sides 
are scattering, fine, short spines. Peritreme long and only slightly 
bent around coxa two and extending forward past coxa one. Anal 
plate separate in both sexes, large, broad, and not pointed behind but 
Figure 96. Seius orchestoideae 
with the posterior margin straight across; all corners rounded. Anal 
opening in the center. Male genital opening on anterior margin of 
sternal plate; no teeth on the legs. 
From the foregoing characters this species according to Banks’s 
key belongs in the genus Seius, which however he says has been 
divided by Ribaga into four sub-genera. As I have not been able 
to obtain Ribaga’s paper, it seems best to place my specimens pro- 
visionally in the genus Seius sens. lat. This species was taken from 
the large amphipod Orchestoidea californiana, which is common on 
the beach. The mites were fastened underneath the body and as 
many as twenty-seven were taken from one amphipod, and great 
