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Show 76 DR. W. G. RIDEWOOD OX THE CRANIAL [May 3, branchiostegal ray, a fact already pointed out by Valenciennes (Hist. Nat. Poiss. xxi. 1848, p. 11). Incidentally it may be pointed out as an item of evidence in favour of regarding the hyomandibular as an element of the liyoid arch, a view which Pollard has contested that there is invariably a definite articulation between the front of the opercular bone and a special process from the back of the hyomandibular, comparable with the less definite articulation between the antero-superior extremities of the branchiostegal rays and the epihyal and ceratohyal. It is generally assumed, and the results of the present investigation tend to show that the assumption is justifiable, that a large number of branchiostegal rays is a primitive character. The greatest number of rays occurs in .Elops, which has from 30 to 35 on each side ; Megalops has about 24 ; and Engraulis, Coilia, and Dussumieria from 10 to 13. Albula has 15, but the allied deep-sea genus Bathythrissa only possesses 6. The commonest numbers are from 6 to 9 : Chanos and Gymnarchus have as few as 4. The jugular plate present in Elops and Megalops is not definitely related to the hyoid arch, but, on the contrary, is united by ligament with the mandibular symphysis. Functionally, at all events, the jugular plate belongs to the branchiostegal series, and it is more convenient to treat it under this heading than elsewhere. Parker's recognition of the urohyal as a basibranchiostegal is altogether erroneous. The urohyal is never superficial and does not support the gill-cover; it is an ossified tendon of the lingual retractor muscles (see " Hyobranchial Series " below). Hyobranchial Series.-The unfortunate application by Parker (Phil. Trans, vol. 163.1873 (1874), p. 101) of the name " basibranchiostegal" to the urohyal bone has occasioned much confusion, and in spite of the unsuitability of the term it continues to be employed by certain writers, and appears in a paper published as recently as 1901 (Supino, Ric. Lab. Anat. Univ. Roma, viii. 3, 1901, p. 18). Even if the urohyal could be shown to belong to the branchiostegal series, the prefix basi- would be misleading as implying homology with the component parts of the copular skeleton, for the branchiostegal rays are dermal, not visceral bones. As a matter of fact, the urohyal is an ossified tendon, or an ossification of the intermuscular septa, as Parker himself admitted, lying between the two sternohyoid muscles +, and is # Anat. Anz. x. 1895. By his describing the Teleostean skull as metautostylic (p. 25) I conclude he regards the hyomandibular as a part of the mandibular arch ; at all events he considers the hyomandibular of the Teleostean to be homologous with the prespiracular cartilage of Sharks, which, from its position in front ot the spiracle, is undoubtedly a constituent of the mandibular and not of the hyoid arch. + Gegenbaur (Morph. Jalirb. iv., Suppl. 1878, p. 17) writes :- " Er dient, vvie sonst, zur Insertion der subbranchialen Muskulatur, und hat weder zur Membraua branchi-ostega noch zu den Kiemenbogen irgend eine Bezieliung." See also Vetter Jena. Zeitschr. xii. 1878, pi. xiii. fig. 1 0 ,8th. Gunther (‘ Study of Pishes,' 1880, p. 91) says that the urohyal " separates the musculi sternohyoidei, and serves for an increased surface of their insertion." |