With reference to the fossil Dipnoi, it may be stated that, so far as they are known, the cranial roofing bones are more numerous than in the existing genera, and they cannot readily be compared with those of the latter, or with the numerically reduced and more definitely arranged bones of most Teleostomi. There is also evidence that in some fossil Dipnoi (e.g. _Dipterus_) the chondrocranium and the mandibular suspensorium (palato-quadrate) must have been replaced by cartilage bones to an extent which has no parallel in any of the surviving types.[208] Jugular bones were present in _Dipterus_ and _Phaneropleuron_. {234}MEDIAN FINS AND APPENDICULAR SKELETON [Illustration: FIG. 135.—The cartilaginous radialia of the first dorsal fin of _Mustelus antarcticus_. (From Mivart.)] THE MEDIAN FINS.—Whether existing in the form of a continuous fin, or as discontinuous isolated fins, the median fins are provided with skeletal supports, and also with muscles, primitively formed from intrusive clusters of cells derived from a variable number of the neighbouring myotomes, for their varied movements. The skeletal structures of the dorsal and anal fins consist of a series of bony or cartilaginous, rod-like, and typically tri-segmented radial elements or pterygiophores,[209] supporting distally a series of dermal structures in the shape of numerous slender horny fibres or ceratotrichia, as in the Elasmobranchii and Holocephali, or a smaller number of bony dermal fin-rays, which are probably modified scales or lepidotrichia,[210] as in the Teleostomi. The typical tri-segmented character of the radialia is often retained in many existing Elasmobranchs (Fig. 135) and in _Pleuracanthus_, in _Neoceratodus_ amongst the Dipnoi, in the Chondrostei, in existing Holostei (Fig. 136), and to a greater or less extent in several families of Teleosts (_e.g._ Salmonidae, Esocidae, Cyprinidae, and some Acanthopterygii); but in the latter group the radialia are greatly prone to reduction, and hence they are more generally bi-segmented, and sometimes consist of a single proximal segment only (e.g. _Gymnotus_). In all these Fishes the proximal segments are the longest and the most persistent, and when reduction occurs it is at the expense of the middle and distal segments. {235}[Illustration: FIG. 136.—The tri-segmented radialia and the fin-rays of part of the dorsal fin of _Amia calva_. _p.s_, _m.s_, and _d.s_, The proximal, middle, and distal segments of a radial; _f.r_, fin-rays. (From Bridge.) FIG. 137.—The first four radialia of the dorsal fin of _Mesoprion gembra_, showing the chain-links for the ring-like bases of the fin-rays. _r.e^1_, _r.e^4_, First and fourth proximal radialia.]