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Show 584 DR. W. G. RIDEWOOD ON THE HYOBRANCHIAL [May 18, cartilage. On the external edge of the first ceratobranchial these processes are of considerable length and form a series of five or six. Similar, but more irregular, cartilaginous processes stand out from the postero-internal border of the fourth ceratobranchial and from the external border of the commissural cartilage. If, now, these irregular finger-shaped processes of the first ceratobranchial were confluent laterally so as to form a continuous curved wall, and if the same coalescence occurred with regard to the processes of the commissural cartilage and fourth ceratobranchial, the whole branchial skeleton would assume the form of two approximately hemispherical cartilaginous basins perforated at the bottom by three oblique slits-a form of branchial skeleton which is peculiar to Xenopus (16). Which of the two forms is the more primitive, that with the irregular palisade, or that wdth the continuous marginal wall, it is difficult to say ; but I a m inclined to regard the branchial skeleton of the early Xenopus larva as more primitive than that of any other Anuran larva yet described l. The fourth branchial cleft (the last of the three perforations in the branchial cartilage) is shorter than the two preceding. The distal end of the third ceratobranchial is exceptionally broad, and is characterized by the constant presence of either a deep notch or a foramen. The proximal end of the first ceratobranchial is broad, and merges indistinguishably into the cartilage of the hypobranchial plate. It bears a backwardly directed spicule of cartilage (sp\ fig. 1). The second ceratobranchial (cbr2, fig. 1) is bound by connective tissue to tbe posterior surface of the first ceratobranchial, to the median side of this spicule. The proximal part then curves upwards and backwards, and giving off a backwardly directed spicule of its own, diminishes in thickness and becomes continuous with the cartilage of the proximal end of the third ceratobranchial, just w-here the latter unites with the hypobranchial plate ". The third ceratobranchial curves over in a similar manner, and has its own spicule (sp3, fig. 1). The fourth ceratobranchial is broader and far more irregular in shape than the other three. It curves up, is confluent with the hypobranchial plate, and sends backwards a curved cartilaginous process, which may possibly represent the spicule of this arch. It should be noted that the whole of the cartilage of the branchial plate is continuous throughout. This continuity is obvious at the distal ends of the arches, but, on account of the sharp curvature of the 1 Schulze (18. p. 11) and Naue (9. p. 14) seem to regard the front wall belonging to the first ceratobranchial as a thin, continuous, concave plate of cartilage in the forms of Anura which they respectively studied. The plates are probably subject to a great amount of individual variation, but in none of the numerous larva? which I have examined (Rana, Bufo, Alytes, Pelobates, Pelodytes, Hyla) have I found the wall nearly so continuous and entire as in Xenopus. 2 I fail to see the force of Gaupp's argument (5. p. 403) for considering the parts of the first two branchial bars lying internal to their spicules as belonging to the hypobranchial plate rather than to the ceratobranchials (branchialia) themselves. |