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Show 590 PROF. G. B. HOWES ON THE CORACOID [June 20, young of members of six orders of Placental Mammals (see the list given below). I give figures of some of m y specimens, and it will be noticed that there is evidence of independent parallelism of reduction of tbe bone named. The conditions suggest that the differences between the Gholoepus and Bradypus figured may be akin to those between Gehus and Homo, Lepus and Sciurus, and that the Edentata may be on a variational equality with other orders in respect to the reduction in question. To turn finally to the wrell-known overgrowth of the Edentate epicoracoid and scapula, for enclosure of the so-called coraco-scapular foramen. Lydekker merely alludes to the similarity in this respect between the Edentates and Dicynodonts. So far as I am aware, this peculiarity is invariable only in certain Edentata and Cebidae1, among living Mammals, and a similar condition is well known to occasionally occur in Man. Prof. Bland Sutton has instituted comparisons2 between the human blade-bone thus modified and that of the Sloth, and in so doing he has remarked3 "I a m disposed to the view that the transverse ligament in M an is the fibrous representative of this bony bridge constant in Sloths, and that the occasional occurrence of a complete osseous foramen in this situation is not to be regarded as an ossification of the transverse ligament, but as a reversion to a former condition." The known facts of morphology lend no support whatever to this view. Were it tenable, the embryonic scapula of M a n should bear an expanded if not an actually perforated prescapular lamina, which it does not4. The entire absence of the prescapular lamina in the Monotremes and Anomodontia, and the fact of its known increase of expansion during development in M a n and in some few other Placentalia, go far towards proving that its overgrowth to meet the epicoracoid must be in all cases secondary ; and they testify to an independent parallelism of modification in the two great classes of animals. The condition occasionally met with in M a n may be closely paralleled by the Tapir among placental quadrupeds. The Bradypodines are remarkable for the secondary association of the clavicle with the coracoid (see above, p. 586). In the Choloe-pines the apex of the acromion becomes inwardly rotated, and, together with the clavicle and coracoid, bound up in a dense fibrocartilaginous mass5. In Cycloturus the scapula differs from that of all other Edentata but some Arrnaddlos (ex. Basypus minutus) in the inward rotation of its antero-ventral border. As viewed from the front (see fig. 2 b, p. 591), this is very conspicuous. I find, on examination, that this peculiarity is associated with the presence of a very powerful ligament (lg.) which passes between the body of the adult acromion and the scapula, enclosing a foramen above. In 1 In Ateles marginatus (fig. 1 h) (not in A. melanochir), Brachyteles, and Lagothrix. 2 ' Ligaments, their Nature and Morphology.' London, 1887. 3 Op. cit. p. fi. 1 Cf. Parker's Ray Soc. Monogr. pi. xxx. figs. 9 and 12. 5 Cf Parker, op. cit. pi. xxi. figs. 10 and 22. |