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Show 588 FROF. G. B. HOWES ON THE CORACOID [June 20, II. As to the Mammalian Coracoid. A leading feature in Mr. Lydekker's paper is the presumed demonstration that in Bradypus the " so-called epicoracoid enters to a small extent into the formation of the glenoid cavity." He urges this, in forcible opposition to an assertion of m y own that " the exclusion of this element from the glenoid facet is one of its most characteristic features ;" and from the context of his paper the reader would be prone to conclude that I had laid this down as a condition characteristic of all Mammals. In m y original paper, to which the assertion he transcribes is but a casual allusion, I expressly stated x that in the higher Placentalia the so-called epicoracoid " comes to enter into the formation of the glenoid facet in proportion as the coracoid bar is suppressed," adding that " the latter structure retires from the scene as the 'coracoid epiphysis' of human anatomists." Mr. Lydekker asserts that in both the Dicynodont and the Bradypodine the " so-called epicoracoid enters to a small extent into the formation of the glenoid cavity;" but while his figure of the former depicts it as contributing an altogether insignificant share in the cavity, that of the latter represents it as contributing wellnigh one linear half of it. There is an incongruity here; and in proceeding to deal with it I incorporate some observations upon the Mammalian coracoid which have accrued since m y former paper was written. M y friend Mr. Oldtield Thomas has generously allowed me to examine the material which passed through Mr. Lydekker's hands. The latter gentleman infers that the coracoid of Sloths consists of but one element (his so-called coracoid, Co', see figs. 1 d, 1 e), therein implying that that element which I have claimed as the homologue of the Monotreme's coracoid (his metacoracoid) is in them absent. There is in our National Collection a blade-bone of Cholcepus didactylus (fig. 1 e) in which both coracoidal elements are well represented; and it will be noted that the epicoracoid (Co') is completely excluded from any share in the glenoid facet, like that of both the Monotreme and the Rabbit, on comparison of which I originally sought to reduce the pectoral girdle of all Mammals to a uniform plan of structure. I find the metacoracoid (Co"), which effects this exclusion, represented in a very young Bradypus cuculliger by a feebly constituted fragment of bone (Co", fig. 1 d) wedged in between the epicoracoid and scapula. The epicoracoid in this specimen is remote from the actual glenoid border, there being present at this period of growth a considerable tract of cartilage (the dotted area of fig. 1 d), into which the epi- and rueta-coracoidal centres are alike free to extend. The condition of this specimen is very nearly that of the Rabbit (fig. 1 a) when first the metacoracoid (Co") appears. The period of independent duration of this bone is, in the Placentalia, 1 Journ. Anat. & Phys. vol. xxi. p. 193. |