OCR Text |
Show 1904.] OSTEOLOGY OF THE ELOPfD.E AND ALBULID/E. 41 low down, and far in advance of the endosteal articular, i. e. not as in Albula. The articular facet for the head of the quadrate is formed in its upper part by the endosteal articular, and in its lower part by the angular bone, which is fused with the ectosteal articular. Hyopalatine Series (text-fig. 10, p. 40).-The hyomandibular articulates with the cranium by two heads, the anterior one being smaller and standing out more distinctly than the other. The palatine articulates with the ethmoid region by a single head, so far as can be seen in the dried skull. The symplectic makes with the lower limb of the hyomandibular an angle of 115 or 120 degrees. Minute teeth occur on the palatal surfaces of the palatine, ento-pterygoid, and ectopterygoid bones. Opercular Series (text-figs. 9 & 10, p. 40).- -The shape, size, and relations of the opercular bones are so clearly shown in the figures that no description is necessary. The branchiostegal rays are 35 in number in one specimen, which may be called " specimen A." The first one is attached just beneath the anterior end of the ceratohyal * ; those that follow form a closely-set series along its ventral border, while the last fourteen are affixed to the outer face of the epihyal, and gradually become larger and flatter as one traces the series backwards. The last three extend farther forward over the outer face of the epihyal than the others, which has the effect of making the transition to the subopercular and opercular bones a very gradual one. In no modern form is it more clearly seen than in Elops that the opercular and subopercular are the two terminal elements of the branchiostegal series. In a larger specimen (B) the numbers are less-20 on the ceratohyal and 12 on the epihyal. In specimen C there are 29 rays on the right side and 31 on the left; in I) there are 28 on each side : but in each of these specimens there are probably some rays missing. A median jugular plate is present, attached by ligament to the back of the mandibular symphysis (text-fig. 10, j, p. 40). Hyobranchial Series (text-fig. 11, p. 42).-The interhyal is ossified. The epihyal is relatively large. The upper and lower hypoliyals are approximately equal in size. The glossohyal is a flat cartilage, horizontally disposed, with a minute endosteal nodule in its posterior end, and bearing on its upper surface a finely dentigerous membrane-bone. The urohyal is long, and extends back as far as the anterior extremity of the fifth ceratobranchial. A long, finely dentigerous bone overlies the three basibranchials. The third basibranchial has degenerated in the anterior five-sixths of its length into a fatty mass, but the hinder one-sixth is a small, fairly compact bone partially covered by the long dentigerous bone just mentioned, the rest of its upper surface being covered by small, uncoalesced, readily removable dentigerous plates. The cartilage-plate representing the undifferentiated fourth and fifth * The occurrence of branchiostegal rays along the whole length of the ceratohyal, up to its anterior extremity, is to be regarded as a primitive feature, even more primitive than the greatness of the number of the rays. Amin is more specialised than Elops in this respect. |