ANATOMY OF MEGAPTERA LONGIMANA. 33 



of the cavity is proximal to the middle of the node ; this is very 

 marked in both the nodes of the index digit, while in the more 

 distal nodes it is rather distal to the middle of the node. On 

 dividing the perichondrium the cavity opens freely, exposing 

 the two smooth synovial surfaces, and extends over the whole 

 of the ends, except in the three distal nodes of the two great 

 digits (III. and IV.) and the distal node of digit V., in which 

 it does not reach quite to the extensor surface or fully to the 

 sides, especially not on the side to which the node projects 

 most. But the depression indicating the line of the joint is 

 seen equally on the extensor surface. The part not separate is 

 occupied by soft fibrous tissue, showing as a white line on 

 section. As bearing on the incompleteness of diarthrodial 

 articulation in the above-mentioned more distal nodes, it may 

 be remarked that these nodes themselves are flexible, being 

 flat and thin compared with the more proximal nodes, which 

 are very thick. 



Form of the Articular Surfaces. The surfaces are generally 

 slightly concavo-convex, the convexity generally on the proximal 

 cartilage, but the curving is less in the direction between the 

 flexor and extensor aspects than transversely in the direction 

 from digit to digit. This departure from the flat in the inter- 

 phalangeal joints is less in the more proximal nodes. At the 

 metacarpo-phalangeal joints the surfaces are more nearly flat, 

 or are sinuous, varying as follows : The metacarpal is, on the 

 index digit, slightly convex ; on digit IV. slightly concave ; on 

 digits III. and V. it is sinuous, the concavity on the ulnar half 

 in digit III., on the radial half in digit V. The metacarpo- 

 phalangeal joints of digits II. and IV. are nearly opposite to 

 each other in the limb. 



The Digital Joints in Megaptera in comparison with those 

 of other Whales. The synovial cavities are well developed in 

 Megaptera. In my 64-feet-long B. musculus the cavity, indi- 

 cated on both surfaces, extended across only the middle third, 

 or less, of the node, and was absent in the smaller nodes. In 

 Mystieetus there are the surface furrows and there is motion 

 between the two parts of the node, but the boundary, which is 

 seen through and through, is not effected by a cavity but by 

 a layer of very soft cartilage. 



