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Show 480 ON THE CANAL-SYSTEM OF PTERASPIDIAN FISHES. [May 17, of an extensive canal-system, which indicates a more highly specialized development of the "lateral-line" structures than has hitherto been suspected. As shown by the drawing (p. 479), there are four longitudinal canals, two marginal, and two situated close together in the median area of the plate ; and the latter pair diverge in front, perhaps meeting the former at the edge, while more posteriorly they are all connected at irregular intervals by similar transversely-directed passages. The median commissural branches extend directly across the space between the two longitudinal canals they unite; but those proceeding to the lateral canals take a slightly more devious course, being inclined either backwards or forwards. Throughout their extent the tubular excavations give rise to short diverticula, alternately left and right, which place them in connection with the external pores; and these are most numerous in the median portion of the shield and the great marginal trunks, being relatively further apart in the lateral commissures. The complexity and widely-spread character of the system is still more satisfactorily demonstrated in the almost perfect specimen of Holaspis already referred to; and here, it will be observed, there is an essentially similar arrangement. Our fossil corresponds to the central portion of this shield, which appears to consist of the homo-logues of the seven plates of Pteraspis wholly fused together. And it is perhaps worthy of note that neither here nor in any other Pteraspidian have distinct traces of the pores been detected upon the rostral region in advance of the " orbital" notches or apertures. Finally, it is interesting to institute a comparison between these ancient traces of a " lateral-line " system and the various structures adapted for the protection of the corresponding sense-organs in existing fishes. Chimaeroids possess merely the primitive open groove; but in Selachians the canal is complete, and there are also present the short secondary diverticula leading to tbe external pores. The last-named branches, however, are all directed to one side (outwards or downwards) of the main canal in the Rays and upon the trunk of the Sharks ; and there is no very close agreement with Pteraspis even in the cephalic region of the latter group, where the branches are given off in both directions, but are not completely closed, being peiforated by a series of small orifices in addition to the terminal one *. Moreover, it is scarcely likely that these canals in the old Devonian fish had the Selachian mode of development. In bony fishes, where the structures bear a similar relation to the hard skeletal parts, in most cases excavating them, there is naturally a much greater resemblance ; and some of the most specialized Teleostei (e. g. the Pleuronectidse) exhibit an almost precisely corresponding " feather-barb " arrangement2. Unfortunately, however, the character of the sense-organs themselves necessarily remains unknown, for the palaeontologist can rarely give much sure 1 P. C. Sappey, ' Etudes sur l'Appareil Mucipare et sur le Systeine Lyru-phatique des Poissons,' 1880, p. 55, pl. ii. fig. 2, pl. x. fig. 1. 2 P. 0. Sappey, op. cit. p. 44, pl. xi. figs. 3, 4. |
