OCR Text |
Show 1905.] OSTEOLOGY OF THE EURYL.EMIILE. 55 In the Eurylaemidae and Cotingidae-at least in so far as Rupicola is concerned-the major portion is well developed, but the brevis portion has now receded, not extending beyond the middle of the humerus, and having an entirely fleshy insertion ; the longus portion, on the other hand, is slender and terminates in a long tendon. This interpretation of the transformations of the deltoides major et minor, it will be noticed, runs directly counter to that of Dr. Chalmers Mitchell, who, in a paper " On the Anatomy of Gruiform Birds " (6), contended that apocent.ricity in this muscle was shown by the gradual extension down the shaft of the major portion. It would seem, rather, as if the archicentric condition were represented by the maximum downward extension, and that apocentricity is represented by the gradual reduction of muscular tissue. That this reduction and inevitable suppression of the brevis portion represents an extremely specialised condition there can be no doubt; and the fact that it is shared also by the Cotingidae seems to me, coupled with the numerous other points which these two groups share in common, to show conclusively that the Eurylaemidae and Cotingidae must henceforth be regal'ded as very closely related forms. These two groups differ in some other myological characters, as might be expected. The most noticeable is the fact that the latissimus dorsi posterior in the Cotingidae appears to be wanting, though it must be remarked I have only been able to examine a single specimen of Rupicola in this connection. In the Eurylae-midfe both muscles are present, strap-shaped in form, and widely separated; therein differing from the Corvidae, in which they are of considerable size and slightly overlap one another. But this feature is one of many primitive characters which the Corvidae have retained. The peculiar myological resemblances which these birds share do not necessarily imply relationship ; but, as I have just remarked, there are so many structures in which these two groups agree, that it is impossible to entertain any notion of convergent resemblance between the two. The points of likeness are so peculiar, and affect such different, independent systems, that correlated variation and convergence cannot be regarded as a satisfactory explanation of the case. When two apparently convergent forms come to be particularised, each new point of resemblance which is brought to light is to be regarded as an additional link in the chain of evidence, establishing the common origin of the two forms in question. Thus, then, I contend there is no evidence which will justify the present isolated position which has been almost universally assigned to this group during the last few years. It is quite possible that further investigation will show that the Eurylaemidae are entitled to rank no higher than a subfamily of the Cotingidae. |