This interpretation of the meaning of the transformation process is in accordance with all the previous evidence both from the side of the palæostracan as from the side of the vertebrate, for it signifies that a dwindling process has taken place in the foremost of the original prosomatic appendages--the cheliceræ and the endognaths; while, on the contrary, the ectognath and the metastoma have continued to increase in importance right into the vertebrate stage. This process is simply a continuation of what was already going on in the invertebrate stage, for whereas in Eurypterus and other cases {307}the cheliceræ and endognaths had dwindled down to mere tentacles, the ectognath was the large swimming appendage, and the metastoma was on the upward grade from the two insignificant chilaria of Limulus. The transformation of these foremost appendages into a suctorial apparatus is very common among the arthropods, as is seen in the transformation of the caterpillar into the butterfly, and it is in accordance with the evidence that the main mass of that suctorial apparatus should be formed from appendages corresponding to the ectognath and metastoma rather than from the four endognaths. In all probability the _nucleus masticatorius_ of the trigeminal nerve with its innervation of the great muscles of mastication is evidence of the continued development of the musculature of these two last prosomatic appendages, just as the descending root of the Vth demonstrates the further disappearance of all that belongs to the foremost prosomatic appendages. As yet, however, as far as I know, the musculature of the head-region of Petromyzon has not been brought into line with that of other vertebrates, and until that comparative study has been completed it is premature to discuss the exact position of the masticating muscles of the higher vertebrates. The analysis of these tentacular segments belonging to the trigeminal nerve presents greater difficulties than that of any of the other cranial segments, owing to the deficiency of our knowledge of what occurs at transformation. Light is required not only on the origin of the new muscles but also on the origin of the new and elaborate cartilages which are newly formed at this time. Miss Alcock has not yet worked out the origin of all these cartilages and muscles, so that we are not yet in a position to analyze the trigeminal supply in Petromyzon into its component appendage elements, an analysis which ought ultimately to enable us to determine from which appendage-muscles the masticating muscles in the higher vertebrates have arisen. As far as the muscles are concerned, she gives me the following information:-- The tongue-nerve supplies in Ammocoetes the rudimentary muscles which pass laterally from the base of the large ventral tentacle to the wall of the throat, and even in Ammocoetes must possess some power of moving that tentacle.