208 BULLETIN OF THE 
spiral branchiæ of Ceraurus, Plate III., figs. 4-6, are usually larger 
and coarser than those of Calymene, and form the second variation of 
the spiral. > 
The third type of the branchis is shown in Plate III. fig. 2. As 
far as yet known, this is confined to the anterior segments of the 
thorax. 
The epipodite.or branchial arm was attached to the basal joints of 
the thoracic legs and formed of two or more joints. This has been 
called a branchial arm, not that it carrried a branchia, but on account 
of its relation to the respiratory system. It is regarded as an arm or 
paddle, that, kept in constant motion, produced a current of water cir- 
culating among the branchis gathered close beneath the dorsal shell. 
This would be necessitated by the character and position of the branchiæ 
and the evident habits of the Trilobite. The best illustration of this ap- 
pendage, with the setiferous exterior joint, is shown in Plate III. fig. 9. 
Of the modification the respiratory apparatus underwent beneath 
the pygidium, we have no evidence. If we estimate the branchiæ by 
the character of the dorsal shell, we would say that in some genera, 
Remopleurides, Paradoxides, etc., with very small pygidiæ, the 
branchiæ were doubtless aborted or mere rudiments, and that in those 
genera with larger pygidiæ, Asaphus, Bronteus, etc., the branchiæ 
were fully developed as beneath the thorax. 
Itis difficult to conceive how a thin pellicle or membrane that 
served the function of respiration could be preserved as the branchiæ, 
or spirals, as we call them, are found in the Trilobite. It is not 
certain but that these parts, as now found, were the supports of delicate 
branchiæ attached to them; this has objections, as the spirals and 
slender ribbons are comparable to the branchiæ of some species of 
Cyamus, as shown on Plate IV. figs. 9 and 10. In either case they 
are all that is left to show the respiratory apparatus of the Trilobite, 
and in that sense are called the branchie. 
AFFINITIES OF THE TRILOBITE. 
That the affinities of the Trilobite are with Limulus and its allies 
there is no longer any reasonable doubt. The observations of Billings, 
Packard, Dohrn, and other recent writers, have served to establish 
the views of previous authors on the subject, which have been confirmed 
by the discovery of the more important characters of the structure and 
arrangement of the cephalic appendages. 
The classification of the group to which the Trilobite bélongs is 
