. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. DE VILLIERS: ASCAPHUS TRUEI STEJNEGER 1' An additional internal perichondrial lamella is then almost immedi- ately followed by intense enchondral ossification, but still the orbito- sphenoid remains separated from the parasphenoid by a cartilaginous tract, continuous with the permanently cartilaginous base of the skull which is invested by the parasphenoid. The condition is sketched in Fig. 6, which shows that the mutual relations of the bones as figured by 05 TnC. UCuT Fig. 6. Transverse section the orbital region of
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. DE VILLIERS: ASCAPHUS TRUEI STEJNEGER 1' An additional internal perichondrial lamella is then almost immedi- ately followed by intense enchondral ossification, but still the orbito- sphenoid remains separated from the parasphenoid by a cartilaginous tract, continuous with the permanently cartilaginous base of the skull which is invested by the parasphenoid. The condition is sketched in Fig. 6, which shows that the mutual relations of the bones as figured by 05 TnC. UCuT Fig. 6. Transverse section the orbital region of the Ascaphus skull, br = brain; fnt = fontanellar tissue; frp = frontoparietal; mc = marrow cavity; os^ orbitosphenoid; prsph = parasphenoid; ucb = unossified cranial base; ucw^ unossified cranial wall. Other abbreviations as for previous figures. Noble (1931, p. 218) are considerably different from those of the skull microtomised. Noble draws the skull, as if (a) parasphenoid and orbitosphenoid were contiguous laterally and anteriorly, (b) the para- sphenoid were surrounded by the orbitosphenoid anteriorly and (c) the posterior portion of the vomer invested the orbitosphenoid. Of these, (a) only is conditionally true, for in the region of the optic foramen, the orbitosphenoid and parasphenoid actually abut on each other, so that the whole brain case appears osseous externally, except for the tract of fibrous connective tissue separating the two fronto- parietals dorsally. In sections showing the optic nerve actually lying in the optic foramen, the side of the skull is cartilaginous dorsal, and very weakly perichondrially ossified ventral to it. But also this ven-. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Harvard University. Museum of Comparative Zoology. Cambridge, Mass. : The Museum
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