of locomotion. [Illustration: FIG. 61.--Embryos of _Heterodontus japonicus_ Maclay and Macleay, a Cestraciont shark, showing the backward migration of the gill-arches and the forward movement of the pectoral fin. _a_, _b_, _c_, representing different stages of growth. (After Dean.)] "May I express the hope that what I have said is sufficient to show in what a state of uncertainty our views are regarding the morphological nature of the paired fins, and upon what an exceedingly slender basis rest both of the two views which at present hold the field?" As to the backward migration of the ventral fins, Dr. Bashford Dean has recently brought forward evidence from the embryo of a very ancient type of shark (_Heterodontus japonicus_) that this does not actually occur in that species. On the other hand, we have a forward migration of the pectoral fin, which gradually takes its place in advance of the hindmost gill-arches. The accompanying cut is from Dean's paper, "Biometric Evidence in the Problem of the Paired Limbs of the Vertebrates" (American Naturalist for November, 1902). Dean concludes that in _Heterodontus_ "there is no evidence that there has ever been a migration of the fins in the Gegenbaurian sense." "The gill region, at least in its outer part, shows no affinity during proportional growth with the neighboring region of the pectoral fin. In fact from an early stage onward, they are evidently growing in opposite directions." =Kerr's Theory of Modified External Gills.=--"It is because I feel that in the present state of our knowledge neither of the two views I have mentioned has a claim to any higher rank than that of extremely suggestive speculations that I venture to say a few words for the third view, which is avowedly a mere speculation. "Before proceeding with it I should say that I assume the serial homology of fore and hind limbs to be beyond dispute. The great and deep-seated resemblances between them are such as to my mind seem not to be adequately explicable except on this assumption.