OCR Text |
Show 1890.] HELODERMA SUSPECTUM. 225 subcylindrical form ; they articulate with the basihyal posterior to the mesial heads of the anterior cornua. Curving backwards and outwards, their hinder ends are tipped with cartilage, which latter character reminds one of the thyro-hyals as seen in most birds-all ordinary existing birds. It is only the anterior joints of the posterior cornua of the hyoidean arches in this Lizard that ossify ; all the remaining parts of the apparatus'are cartilaginous, even in very old specimens. From this brief description it will be seen that the hyoidean arches in Heloderma simply add another pattern of these structures to the various forms they assume among Lizards generally. According to Cuvier, Hoffmann, the Parker, and many other anatomists, these parts differ in a number of species of the Geckos, in Gonyocephalus, in Lguana, in Scincus, in Chamceleon, and in many other species and genera. In such a species as Lacerta viridis, according to Professor T. J. Parker (' Zootomy '), all three cornua of the hyoid apparatus are present, the anterior, middle, and posterior, and such elements are represented as the hypo-hyal, the stylo-hyal, the cerato-hyal, and the epibranchial of the second branchial arch. On the Shoulder-Girdle and the Pectoral Limb. A description of the simple form assumed by the sternum in Heloderma has already been presented above. This structural simplicity appears to be extended to the shoulder-girdle. A broad part of the mesial border of either coracoid remains cartilaginous, and this is wider in front than it is behind. Fusing with the corresponding scapula, the osseous part of the coracoid at a point upon the posterior margin of the girdle yields to the articular surface of the glenoid cavity its ventral moiety. Just anterior to this point is to be seen a small fenestra, that appears to indicate the original divisional space between the precoracoid and the coracoid proper. In rough outline the form of the coracoid simulates the sector of a circle, the apex being at the glenoid cavity. Anteriorly these bones overlap each other, while posteriorly the mesial margin of either one articulates with the groove occupying the antero-external border of the sternum. In a specimen before me it is the left coracoid that underlaps the right, while the clavicles and interclavicle tend to hold them in this position. It may not, however, be that the left bone is always positioned ventrad. From all this it will be observed that the coracoid in Heloderma having the form described, its several elements are so fused together that it remains only to make out the cartilaginous epicoracoid (mesial rim), the precoracoid and coracoid proper being indicated by the position of a small foramen only, while the mesocoracoid, if it ever exists as a separate ossification in this reptile, is here now completely co-ossified with the other elements. Being rather less than one third the size of the coracoidal portion of the girdle, the scapula has its upper and lower extremities dilated, |