. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. EASTMAN: sharks' TEETH AND CETACEAN BONES. 91 this aspect is identifiable with the petrous bone of other mammals, and has the usual pointed anterior extremity (9). Immediately behind this is seen a tumid shelf-like projection wliich overhangs and partly conceals the anterior process of the bulla, together with its " accessory ossicle," being in fact slightly fused with the latter underneath. This project- ing portion of the front margin is called the process7is anterior petrosi (10) ; and underneath it passe

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. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. EASTMAN: sharks' TEETH AND CETACEAN BONES. 91 this aspect is identifiable with the petrous bone of other mammals, and has the usual pointed anterior extremity (9). Immediately behind this is seen a tumid shelf-like projection wliich overhangs and partly conceals the anterior process of the bulla, together with its " accessory ossicle," being in fact slightly fused with the latter underneath. This project- ing portion of the front margin is called the process7is anterior petrosi (10) ; and underneath it passe
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Image ID: RGFW0D
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. EASTMAN: sharks' TEETH AND CETACEAN BONES. 91 this aspect is identifiable with the petrous bone of other mammals, and has the usual pointed anterior extremity (9). Immediately behind this is seen a tumid shelf-like projection wliich overhangs and partly conceals the anterior process of the bulla, together with its " accessory ossicle," being in fact slightly fused with the latter underneath. This project- ing portion of the front margin is called the process7is anterior petrosi (10) ; and underneath it passes in a longitudinal direction the tensor muscle of the tympanic. Tlie hinder portion of the petrous body forms the posterior process (11), by which, as we have seen, the periotic and tympanic are principally held together. A portion at least of this structure is evidently equivalent to the processus mastoideus petrosi of other Mammals. About midway between the two processes referred to, a sinus occurs in the supero-external margin of the periotic which /6_- '3 -^i^pi. /' 3 I Fig. B. — Left tympano-periotic of the existing Ddphlnapierus leucas Pallas. Superior face, X i- affords ingress by means of an obliquely descending semicircular conduit (12) into the tympanic cavity. This passageway is named by Beaure- gard ductus petro-tympmiicus; by Boenninghaus, who regards it as peculiar to Cetacea, it is called hiatus epitympanicus. Cerebral or inner face (Fig. C). — Viewed from this position, the nar- row inner lip (16) of the bulla appears uppermost in the figure (in reality it is lowermost), and immediately below this, yet separated from it by the continuous tympano-periotic fissure, is the rounded portion of the periotic which contains the cochlea. The windings of the scala may be readily followed, the outer wall being formed by the promontory, and the inner spirals appearing within the large funnel-shaped pit known as theporus acusticus internus{\5), which gives passage for t

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