208 BULLETIN OF THE 



spiral branchire of Ceraiir2is,T\ate 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 branchia; 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 respirator}^ system. It is regarded as an arm or 

 paddle, that, kept in constant motion, produced a current of water cir- 

 culating among the branchiae gathered close beneath the dorsal shell. 

 This would be necessitated by the character and position of the branchiae 

 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 p3'gidium, we have no evidence. If we estimate the branchioe by 

 the character of the dorsal shell, we would say that in some genera, 

 Memopleurides, Paradoxides, etc., with very small pygidise, the 

 branchiae were doubtless aborted or mere rudiments, and that in those 

 genera with larger pygidiae, Asaphics, Dronteus, etc., the branchiae 

 were fully developed as beneath the thorax. 



It is difficult to conceive how a thin pellicle or membrane that 

 served the function of respiration could be preserved as the branchiae, 

 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 

 branchiae attached to them ; this has objections, as the spirals and 

 slender ribbons are comparable to the branchiae of some species of 

 Cyamus, as shown on Plate IV. figs. 9 and 10. In either case they 

 axe all that is left to show the respiratory apparatus of the Trilobite, 

 and in that sense are called the branchiae. 



AFFINITIES OF THE TRILOBITE, 



That the affinities of the Trilobite are with Lbmdus 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 apj)endagcs. 



The classification of the group to which the Trilobite belongs is 



