Beecher (1895 B, p. 309), also from a comparison with _Limulus_, suggested that the eye-lines of _Cryptolithus_, _Harpes_, _Conocoryphe_, _Olenus_, _Ptychoparia_, _Arethusina_, etc., probably represented the optic nerves, and since the eye-lines are usually the main trunks of the dendritic markings, it is fair to assume that he considered the whole as due to branches of nerves. Reed has recently (1916, pp. 122, 173) discussed these lines as developed in the Trinucleidæ, and seems to accept Beecher's explanation. Three explanations of the "nervures" are thus current, and the authors of all of them refer us to _Limulus_ as proving their claims! So far as general appearance goes, the markings on the trilobites more closely resemble the veins of a _Limulus_ than either the nerves or "liver" of that animal. The veins, however, are not in contact with the dorsal shell, but are buried in the liver and muscles, while the arrangement of the arteries, which are dorsal in position, is quite unlike what is seen in the trilobites. The term nervures, as applied to these markings, is not only misleading, but an incorrect use of one of Barrande's words, for by nervures he meant delicate surface markings. Until the real function of the organs which made these markings is definitely established, it may be well to call them genal cæca, for they obviously were open tunnels ending blindly, whatever they contained. The question of the function of the genal cæca can not, in any case, be settled by an appeal to _Limulus_, and it is doubtful if it can be settled at all at the present time. Certain things tend to show that Jacket's explanation is the most plausible, and these may be briefly set forth. Walcott (1912 A, pp. 176, 179, pls. 27, 28) has described specimens of _Naraoia_ and _Burgessia_ in which similar markings are well shown, and where they are obviously connected with the alimentary canal just at the anterior end of the mesenteron. In _Burgessia_, which seems to be a notostracan branchiopod, the trunk sinuses are very wide, and the appearance is on the whole unlike that of any known trilobite. In _Naraoia_, however, the markings are much finer and directly comparable with those of _Elyx_. If my contention that _Naraoia_ is a trilobite should be sustained, it might almost settle the question of the "nervures." In _Burgessia_ these lateral trunks enter the main canal behind the fifth pair of appendages. In the trilobites they debouch much further forward.