42 AFFINITY OF THE TRILOBITES 



SECTION XX. 



The near analogy of the Trilohites to the PhyUopoda appears to me to be especially 

 illustrated by the points of resemblance already alluded to ; viz. the double large eyes, the 

 undeveloped antennse, and the very soft membranous feet. But Bratwhipus seems to be that 

 form among them, with which the organization of the Trilohites has the nearest affinity. In 

 order, however, to render this quite manifest, I have drawn the shell of a Trilobite round 

 the body of a Branchipm, and thus obtained an imaginary form (Fig. 16, Table VI) ; which, 

 I think, will scarcely leave a doubt respecting the near analogy of the two forms. Referring 

 to this figure, the real head will be seen projecting from the cephalic shield, but with this 

 difference in Branchipus that in it the projecting boss forming the head consists of only two 

 divisions, the anterior of which, bearing the antennae and eyes, is smaller than the posterior, 

 to which the branchial apparatus and accessory parts of the mouth are affixed. In Trilo- 

 hites, on the other hand, the projection of the head is either simple or divided into four 

 parts, and in the latter case the first is either the smallest, as in Oleiius, Triartlini-s, Trilobites 

 Sternher(/n, and other allied forms ; or, as is sometimes seen, the first is the largest, the others 

 being all smaller. This proportion indicates very large eyes and antennae, a view which seems 

 verified in the case of the genera enumerated, since even the so-called species of Olenus have 

 small eyes, while Asaphus, lUanus, species of Pliacops, and Oyygia, have very large ones. For 

 the same reason we might also draw an inference from the development of the antennae, 

 and assume that one pair of them perhaps, as in Branchipns, were organs of copulation. 

 I consider myself further justified in this assumption by the structure of the lower side 

 of the head ; for a very large, broad, cephalic shield, such as that seen in Asaphus (Table VI, 

 Fig. 8, a), and Paradoxides (Table I, Fig. 7,) indicates developed organs towards its side. 

 The anterior enlargement beside it (Table VI, Fig. 8, b, h) may perhaps have been the 

 peduncles of linguiforni antennae, or the latter may have been attached to the lateral enlarge- 

 ment beside the cephalic shield (Table VI, Fig. 8, c, c), while the anterior enlargement 

 bore small, short, true antennae, as in Bra/ic/tipi/s. If this conjecture is well founded, we 

 may also easily understand why the anterior division of the head of the Trilobites is so 

 large, sometimes even (as, for instance, in PZ/acops — Division A, — and Asaphus) obliterating 

 the succeeding divisions, and thereby causing an undivided projection of the head. The 

 wide-arched space of the shield beside the projection of the head beneath the eyes, would 

 also be very well calculated for the reception of such linguiforni antennae. The moveable 

 upper lip was unquestional)Iy affixed to the posterior margin of this lower pi'ojection of the 

 head (the real Ch/peiis, Table VI, Fig. 8, d), and the size of the lip depended on the width 

 of this margin, and on the magnitude of the incision. As in Apiis and Branchipus, this lip 

 covered the upper jaw, the form of which perhaps rather resembled the upper jaw of Apus 

 than that of Branchipus, since the hard horny shell also indicates more solid organs of 

 mastication. The third division of the projection of the head, which is generally the largest 

 after the first, probably corresponds with the position of the jaws, and formed the basis of 

 attachment for the muscles. It represents that ring of the body which, in the typical form, 

 is independent, and has the organs of locomotion changed into gills ; and since only a 



