CHAP. III.] 



VERTEBRA AND RIBS. 



103 



5. 



lower vertebrates, Fishes and Snakes, for example, the range of 

 such Variation may be very great. Among Mammals the following 

 may be given as an example of considerable Variation in the 

 number of presacral vertebra? in a wild animal, and such evidence 

 may be multiplied indefinitely. 



Erinaceus europaeus (the Hedgehog). 



Nos. 1—5 in Mus. Coll. Surg., see Catalogue, 1884, pp. 645 and 646; No. 6 in 

 Cambridge Univ. Mus. ; Nos. 7 — 9 in British Museum. 



6. Man. The simplest form of true Meristic Variation in the 

 total number of vertebrae may occur in Man by the formation of 

 an extra coccygeal vertebra, making five coccygeals in addition to 

 five sacrals, i.e. ten pelvic vertebras in all. Instances of this are 

 rare (Struthers), though in many tailed forms such Variation 

 is common. Two cases, in both of which the sixth piece (1st 

 coccygeal) was partially ankylosed to the sacrum, are fully de- 

 scribed by Struthers, J., Journ. Anat. Phys., 1875, pp. 93 — 96. 



In the presence of cases like that last given, there is a strong 

 suggestion that the number of vertebras has been increased by 

 simple addition of a new segment behind, after the fashion of a 

 growing worm : the variation of vertebras thus seems a simple 

 thing. But there is evidence of other kinds which plainly shews 

 this view of the matter to be quite inadequate. Some of these 

 facts may now be offered, and in them we meet a class of fact 

 which will again and again recur in other parts of the study of 

 Repeated Parts. 



Imperfect Division of Vertebrae. 



*7. Python tigris 1 . This is a case of great importance as illus- 

 trating several phenomena of Meristic Division. In a skeleton of 

 Python in the Mus. Coll. Surg., No. 602, the following peculiarities 

 of structure are to be seen. Up to the 147th inclusive the ver- 

 tebras are normal, each having a pair of transverse processes and a 



1 This and the following cases of Pelamis and Cimoliasaurus are discussed by 

 Baub, G., Jour, of Morph., iv. 1891, p. 333. 



