ANATOMY OF MEGAPTERA LONGIMANA. 127 



narrower (except on the two first) ; upper half of shaft broader 

 than at middle ; broadest part lower down, about middle of 

 lower half, while in Megaptera it is on the third quarter of the 

 bone ; taper less towards lower end, from the 2nd to the 7th or 

 8th, than in Megaptera except on its first and last. The 

 breadths of the longest rib of each, the 6th, are, in Megaptera 

 and B. musculus respectively, at external neck, 3| and 3 

 inches ; at middle of upper half, 2| and 3 ; at middle, 2| and 

 2 1 ; at middle of lower half, 2f and 3 ; at 6 inches from lower 

 end, 2f and 3. 



Thickness. The ribs of Megaptera and B. musculus differ 

 mainly in their greater thickness in the former, becoming 

 marked after the 2nd and continuing to the last, as seen in the 

 4th column of the Table. To the grasp, they are ovoids in 

 Megaptera, becoming, relatively to the breadth, thicker as we 

 go back, the four last ribs nearly as thick as they are broad. 

 The measurements given in the Table are taken at the exact 

 middle of the entire rib, which is about the thickest part, but 

 there is not much diminution in thickness till upon the upper 

 and lower fourths of the bone. The upper thinning becomes 

 marked as we approach the angle, and is greater than that 

 towards the lower end. From the 3rd or 4th to the 9th or 

 10th, and again on the last, the thickness to the very end below 

 in Megaptera is very striking compared with the corresponding 

 ends in B. musculus. 



[In B. musculus the beam of the 2nd rib is external, the inner 

 -| thin and deeply grooved on both surfaces ; this along about the 

 middle half of the length of the shaft. The 3rd rib the same, but 

 the grooving less marked. On and after the 4th, the thickest part is 

 at, or a little internal to, the middle of the breadth, with about equal 

 sloping on each side of it. The 13th becomes thin externally, the 

 14th and 15th are thin across their whole breadth. On the typical 

 ribs the beam projects chiefly on the external surface, beginning at 

 the angle, passing obliquely to about the middle of the surface, and 

 ceasing near the lower end. The internal part of the upper half of 

 the shaft is strengthened by a minor beam seen on the inner surface, 

 beginning at the inner border and reaching obliquely outwards to 

 the main beam. The inner surface, taken generally, is flatter than 

 the outer surface, especially along the lower half. At the posterior 

 border a special sub-costal groove and ridge are present towards the 

 lower end, from the 5th to the 12th, beginning 10 to 12 inches from 

 the lower end of the rib and extending upwards for about the same 



