ANATOMY OF MEGAPTERA LONGIMANA. 143 



until we reach the 11th and 12th of B. musculus, which in this re- 

 spect closely resemble the 7th and 8th of Megaptera.] 



This approximation of the lamina above results from the 

 greater breadth of the articular surface in Megaptera, The 

 measurements of this surface on the 4th are, in Megaptera, 

 breadth 2 inches, length 5; in B. musculus, breadth If inches, 

 length, 4J. The surface in Megaptera is elliptical, with the 

 greatest bulge on the inner side. It is bevelled before and 

 behind so as to present two facets ; the anterior rests on the 

 vertebra to which the chevron bone belongs, and is the more 

 flattened of the two ; the posterior rests on the intervertebral 

 disc ; in Megaptera it could not reach the vertebra behind, 

 owing to the great length of the discs. 



[In B. musculus the articular surface, besides being narrower, has 

 a different form, the inner side concave on the three first, nearly 

 straight on the next two, convex behind on the next three, and on 

 the four posterior the surface becomes elliptical but not so broad as 

 in Megaptera. The posterior facet, separated from the anterior by 

 a middle rounded part, may rest on the vertebra behind. The arch 

 (included space) is about as large as in Megaptera.] 



The spines are but little developed in height or in breadth 

 (antero-posteriorly) compared with those of B. musculus. This 

 is, at least in part, owing to the less advanced ossification in 

 Megaptera, but the lower edge of the chevron spines are un- 

 finished in both. This edge has very little of the convexity 

 which is so marked nearly all along the series in B. musculus, 

 giving them the semicircular form in the latter. The 2nd 

 alone in Megaptera shows much convexity, the 3rd and 7th a 

 little ; the 4th is, on the whole, the best developed of the 

 chevron spines. In the subjoined table of measurements of 

 the chevron bones of Megaptera, the 4th of B. musculus is 

 given for comparison. 



Individual Chevron Bones. — The ^irs^ presents two separate 

 triangular laminoe, 3| inches in height, about 3 inches broad at 

 the top, the point directed downwards and forwards. The ninthiB 

 smaller than the 1st, its more blunt point directed straight down. 

 The laminae of the tenth are oval, Ij inch in height. If antero- 

 posteriorly. Their articular surface, however, is very distinctly 

 marked above, and reaches some way down on the inner side, 

 very sharply marked off from the smooth oval internal surface 



