OCR Text |
Show 1014 MESSRS. B. C. A. WINDLE AND F. G. PARSONS ON [Dec. 19, 22. The pectineus is often double. 23. The adductor cruris is single. 24. The semitendinosus has only an ischial head. 25. The femoral head of the flexor cruris lateralis is absent. 26. The tenuissimus is present. 27. The extensor longus digitorum has a femoral origin. 28. The extensor brevis digitorum is always tarsal in origin. 29. The peroneus longus has no definite femoral origin, its tendon passes across the sole. 30. The gastrocnemius has fabellae. 31. The plantaris is present. 32. The flexor brevis digitorum is continuous with the plantaris in the sole. 33. The tibialis posticus is double. 34. The accessorius pedis is fibrous. With the object of rendering comparison more easy we have arranged some of the more important muscles in a tabular form (p. 1015). We have now to consider what lessons may be learnt concerning the relations and systematic position of the animals included in the order of the Edentata from the study of tbe muscles. Flower (Proc. Zool. Soc. 1882, p. 358), in a paper on the mutual affinities of the animals composing this order, says that " the two Old-World forms Manidce and Orycteropodidce are so essentially distinct from all the American families, that it may even be considered doubtful whether they are derived from the same primary branch of mammals, or whether they may not be offsets from some other branch, the remaining members of which have been lost to knowledge." In using the muscles in the endeavour to deal with this problem, the first consideration necessary is to ascertain whether there are any departures from the generalized arrangement of mammalian muscles which are common to all the families of this so-called order, for, if such exist, they are not likely to be adaptations to similar conditions of life in animals far removed in relationship. For instance, if similar wanderings from the generalized mammalian arrangement of muscles can be found in the Pangolin and the Sloth, these wanderings are more likely to be the result of kinship than of an adaptive modification to meet similar conditions of life, for few animals more dissimilar in their habits could be imagined than these two. Everyone who has worked at Edentate myology will at once think of two curious muscular modifications which are not usually found elsewhere amongst the Mammalia, namely, the rectus thoracis lateralis and the femoral head of the flexor cruris lateralis, or biceps. Both these muscles are present in the two families, although, so far as we know, the rectus thoracis lateralis is never found as a distinct muscle outside the order with which we are now concerned, whilst the short head of the flexor cruris lateralis is only to be seen in the Edentates, Platyrrhine Monkeys, and Anthropoids. There are other peculiarities |