. Bulletin of the British Museum (Natural History), Geology. REVISION OF ACTINOPTERYGIAN AND COELACANTH FISHES 357 cephalic division posteriorly, but commences in the parietal and runs down through the frontal, then into the nasal, where it turns sharply back into the antorbital to join with the infraorbital canal. There is just the semblance of a connection between the infraorbital and the supraorbital canals posteriorly, a line of irregular pores passing from the dermosphenotic to the supraorbital canal in the posterior frontal region. Where the sensory canal traverses the nasal it gives off
. Bulletin of the British Museum (Natural History), Geology. REVISION OF ACTINOPTERYGIAN AND COELACANTH FISHES 357 cephalic division posteriorly, but commences in the parietal and runs down through the frontal, then into the nasal, where it turns sharply back into the antorbital to join with the infraorbital canal. There is just the semblance of a connection between the infraorbital and the supraorbital canals posteriorly, a line of irregular pores passing from the dermosphenotic to the supraorbital canal in the posterior frontal region. Where the sensory canal traverses the nasal it gives off a branch anteriorly, which passes forward into the postrostral. This represents the rostral commissure. The bone I have called the postrostral, from its position and extent, represents, then, a fusion of two elements, the rostral and postrostral. In Acentrophorus this ossification is large and separates the nasals widely. In the early Semionotids such as Semionotus capensis from the Cave Sandstone, this bone is reduced and posteriorly. Fig. 72. Acentrophorus varians (Kirkby). Caudal fin. From Gill (1923ft). forms only a narrow strip separating the nasals (and overlies the posterior extensions of the premaxilla). Finally, in the later Semionotids such as Lepidotes and Dapedium it no longer separates the nasals but is reduced to a small ossification anterior to them. This small ossification is termed a rostral by Rayner (1948, fig. 10) and Schaeffer & Dunkle (1950 : 7), and is probably a correct interpretation, since the postrostral portion will have been the region to have undergone the reduction. From these two descriptions it is obvious that the earliest Semionotid, Acentro- phorus, differs markedly from the Parasemionotidae. In the Parasemionotidae the nasals are paired and meet in the mid-line, with a small rostral element between their anterior extremities and the premaxillae. The condition in Acentrophorus has already been described, wherein a large median postrostral
Size: 1725px × 1448px
Photo credit: © Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookauthorbritishmuseumnaturalhistory, bookcentury1900, bookcoll
