appendifers and other rugosities for the attachment of tendons and muscles. It is conceivable that this migration backward of the mouth began very early in the history of the race, and that even before Cambrian times, the antennæ, probably originally biramous appendages like those on the remainder of the body, had dwindled away and become lost. If this is the case, then the first pair of biramous appendages of _Triarthrus_ would be mandibles, the second pair maxillulæ, and the third pair maxillæ. There remain the last pair of cephalic appendages, and they bring up the whole head problem of the trilobites. Beecher has stated (1897 A, p. 96) his conviction that the head of the trilobite is made up of five segments, representing the third, fourth, fifth, sixth, and seventh neuromeres of the theoretical crustacean. As a matter of fact, he really made up the head of seven segments, since he stated that the first neuromere was represented by the hypostoma and the second by the epistoma and free cheeks. Jaekel (1901, p. 157) nearly agreed with Beecher, but made eight segments, as he saw five segments in the glabella of certain trilobites. In his table (p. 165) he has listed the segments with their appendages as follows: 1. Acron, with hypostoma; 2, rostrum (epistoma), with free cheeks; 3, first frontal lobe, with (?) antennules; 4, second frontal lobe, with antennæ; 5, mandibles; 6, first, or pre-maxillæ; 7, second maxillæ; 8, occipital segment with maxillipeds. Jaekel refused to believe that the antennæ of trilobites were really entirely simple, and so homologized them with the antennæ and not the antennules of other Crustacea. In this he was obviously incorrect, but it accounts for his homology of the remainder of the cephalic appendages. It is, at present, impossible to demonstrate the actual number of somites in the cephalon of the trilobite, but I believe that Beecher was correct in holding that the glabella was composed of four segments. There are, it is true, a number of trilobites (Mesonacidæ, Paradoxidæ Cheiruridæ, etc.) which show distinctly four pairs of glabellar furrows, indicating five segments in the glabella. This is, however, probably due to a secondary division of the first lobe. The correspondence of the five segments on the dorsal side with the five pairs of appendages makes it unlikely that any pair of limbs has been lost. The condition in _Marrella_, where a trilobite-like cephalon bears five pairs of appendages, the second pair of which are tactile antennæ, is favorable to the above interpretation. In spite of the apparent degeneration of the first two pairs of appendages in _Calymene_, no limbs are actually missing, and if some are dropped out in the later trilobites it would not affect the homology of those now known. I therefore agree with Beecher in homologizing the appendages, pair for pair, with those of the higher Crustacea. FUNCTIONS OF THE APPENDAGES. _Antennules._