_Cryptolithus_ presents still another and very peculiar development of the endopodites where ability to get purchase on the sea floor is obtained by a stout limb of slight flexibility, bowed and turned backward in the middle, where an enlarged segment insures stiffness. The segments are flattened, and since the greatest strength when used in pushing and crawling is in the long axis of the oval section of the flattened limb, it seems probable that these limbs did not hang directly down, with their sides vertical, but that their position in life was very much the same as that in which they are preserved as fossils. By moving these bowed legs forward and backward in a plane at a small angle to the surface of the body, a powerful pushing impetus could be obtained. They may, however, have occupied much the same position as do those of _Limulus_. In the case of the endopodites, therefore, it is necessary to study the structure and probable method of their use in each individual genus before suggesting what was the probable position in life. In the act of swimming, the position was probably more uniform. When the endopodites were used in swimming, as they undoubtedly could be with more or less effect in all the trilobites now known, those with flattened surfaces probably had them at such an angle as to give the best push against the water on the back stroke, while on the forward stroke the appendage would be turned so that' the thin edge opposed the water. The great flexibility of attachment would certainly permit this, though unfortunately nothing is as yet known of the musculature. The coxopodites of course had less freedom of movement in this respect, and probably could not turn their faces. For this reason, it seems to me likely that those coxopodites which are compressed did not stand with their flattened faces vertical, but in a position which was nearly horizontal or at least not more than 45 from the horizontal. If the flattened faces were vertical, they would be in constant opposition to the water during forward movements and would be of no use in setting up currents of water toward the mouth, as every back stroke would reverse the motion. The position of the exopodites in life seems to have been rather uniform in all the genera now known. I have set forth on a previous page my reasons for thinking that they took little part in swimming, and I look upon them as being, in effect, leaf-gills. It seems probable that in all genera the exopodites were held rather close to the test, the shaft more or less rigid, the filamentous setæ gracefully pendent, but pendent as a sheet and not individually, there having been some method by which adjoining setæ were connected laterally. Free contact with the water was thus obtained without the mingling of endopodites and exopodites which would have been so disastrous to progression. PART II. Structure And Habits Of Trilobites. INTERNAL ORGANS AND MUSCLES.