| OCR Text |
Show 1904.] OSTEOLOGY OF THE ELOPID.E AND ALBULID/E, 77 attached by two ligaments to the right and left hypohyal bones, except in the Mormyroid fishes, in which it is rigidly fixed beneath the anterior part of the copular skeleton. Shufeldt (Rep. U.S. Com. Fish. 1883 (85), p. 820) writes that the urohyal " lies between the sternohyoid muscles, and is not always present where a glossohyal exists." The latter part of the quotation is, I think, open to serious question; it is probably always present in Teleostean fishes. Brooks (Proc. Roy. Dubl. Soc. n. s. iv. 4, 1884, pp. 180-183), in his description of the skull of the Haddock, while adopting the name basibranchiostegal for this tendon-bone, misapplies the term urohyal to that cartilage which lies between the last two pairs of ceratobranchials, and represents the fourth and fifth basibranchials. In connection with the urohyal, it is of interest to observe that in Notopterus there are, in addition to the urohyal proper, a pair of tendon-bones of similar character to it, but of smaller size, projecting backwards from the posterior end of the ventral surface of the second basibranchial. Such tendon-bones are also present in Osteoglossum, Heterotis, and the Mormyridae; but in these fishes they are confluent with the reduced second hypobranchials. In dealing with the homologies of tendon-bones, one must ever be prepared to admit the possibility of convergence; thus, while in Notopterus there are separate tendon-bones related to the second hypobranchials and the second basibranchial, in Diodon (Dicotyl-ichthys) a pair of exactly similar bones project down from the mesial ends of the third ceratobranchials, and represent either the downwardly directed and greatly elongated third hypobranchials, or a pair of tendon-bones confluent with the third hypobranchials, or simply a pair of tendon-bones, the third hypobranchials being absent. In Polypterus, again, the urohyal as a median bone is wanting, but a pan of tendon-bones project downward and backward from the ventral surface of the anterior end of the ceratohyal. The hypohyal of each side of the head is usually double, consisting of distinct upper and lower ossifications. The right and left upper hypohyals are separated by the first basibranchial or the glossohyal, or both, but the two lower hypohyals are articulated together in the median plane. The lower hypohyal in most cases is larger than the upper, but in Hyodon, Elops, Megalops, and Albula the upper and lower hypohyals are approximately equal in size. In Arapaima, Heterotis, Osteoglossum, Petrocephalus, and Notopterus there is but one hypohyal on each side, and this would appear to represent the upper one. This is rather odd when considered in relation with the probability that the single hypohyal of Amia represents the lower of the two. In Mormyroids other than Petrocephalus no hypohyals are recognisable. It is difficult to understand why Supino, in his recent work on the skull of deep-sea Teleosteans (Ric. Lab. Anat. Univ. Roma, viii.-ix. 1901-2), and Starks, in his description of the Serranoicl |
