In the Bohemian _Trinucleoides reussi_ (Barrande), the oldest of the trinucleids, the hypostoma is very commonly present, and is of the proper size to just cover the cavity of the glabella, seen from the lower side, and has, toward the anterior end, side flaps which reach out under the prominent glabellar lobes. This large size of the hypostoma would cause the antennules to be attached outside the dorsal furrows, and the position in which they are attached in the American species of _Cryptolithus_ may be explained as an inherited one, since with the small hypostoma they might have been within the glabella, as in _Triarthrus_. The antennules are seen in three specimens, and in all cases are directed backward. The particular course in which they are drawn in the restoration is purely arbitrary. The second pair of cephalic appendages are represented as directed downward and forward, since in one or two specimens fragments of forward-pointing endopodites were seen near the front of the cephalon, and because in other trilobites the second pair of appendages is always directed forward. The remaining three pairs have a more solid basis in observed fact, for the two or three specimens retaining fragmentary remains of them indicate that they turn backward like those on the thorax, and that the individual segments are longer and more nearly parallel-sided than those of the more posterior appendages. The gnathites of all the cephalic appendages are admittedly purely hypothetical. None of the specimens shows them. As drawn, they are singularly inefficient as jaws, but if, as is suggested by the casts of the intestines of trinucleids found in Bohemia, these trilobites were mud-feeders, inefficient mouth-parts would be quite in order. [Illustration: Fig. 20. _Cryptolithus tessellatus_ Green. A restoration of the appendages drawn by Doctor Elvira Wood from the original specimens and from the photographs made by Professor Beecher. × 9.] The appendages of the thorax and pygidium can fortunately be taken quite directly from the photographs of the dorsal and ventral sides of well preserved specimens. There is of course a question as to the number and the exact form of those on the pygidium, but I think the present restoration is fairly well justified by the specimens. As would be expected from the narrow axial lobe, the gnathobases of the coxopodites are short and small. Summary on the Ventral Anatomy of Trilobites. COMPARISON OF APPENDAGES OF DIFFERENT GENERA. Since the appendages of _Triarthrus_, _Cryptolithus_, _Neolenus_, _Calymene_, and _Ceraurus_ are now known with some degree of completeness, those of _Isotelus_ somewhat less fully, and something at least of those of _Ptychoparia_, _Kootenia_, and _Acidaspis_, these forms being representatives of all three orders and of seven different families of trilobites, it is of some interest to compare the homologous organs of each.