Wilder.] 318 [June 21, 



ing the special homology between the twelve indical phalanges of 

 Gldbioceplialus melas and the three of an ordinary mammal. 



5. It might be thought that such lack of .special homology be- 

 tween the cetacean digits and that of other Mammalia, indicated the 

 propriety of regarding the former as forming a subclass ; but this at 

 once brings up another consideration. 



NUMBER OF VERTEBRAE. 



The number of vertebras (excepting the cervical) , differs greatly 

 among the ordinary Mammalia, as is stated in all works upon com- 

 parative anatomy; from various authorities, chiefly Owen, 63, 2, and 

 Flower, 71, 1 have prepared a table showing the number of cervical, 

 thoracic, lumbar, sacral and caudal vertebras of many species of 

 Mammalia, (105 species representing 91 genera) ; the cervical 

 vertebras are seven in all excepting in Manatus (6) and Cholcepus 

 Hoffmanni (6) and Bradypus tridactylus (9) ; but there is evidently 

 room for different interpretations of the facts in these cases. 



The same is the case with the enumeration of the sacral and cau- 

 dal vertebras, but the variations in their number are so great and so 

 generally recognized that a tabular statement is not required in this 

 connection. I wish here, however, to ask whether the immense 

 elongation of the tail in many species is primordial or secondary; 

 and if the latter, whether the increase is by gradual development of 

 new segments or by the increase in size of some which are formed 

 all together at the front; upon the answer to this question, might be 

 based a discrimination between the segments which immediately 

 succeed the sacrum, and have the structure of vertebras, and those 

 more simple cylinders of bone which have no claim to the title of 

 vertebra beyond their serial relation to the former. 



In any case, the numerical variation of a peripheral part like the 

 tail, would not have a greater morphical significance than that of the 

 phalanges. 



But with the so-called trunk vertebrce the case is very different ; 

 they are the central portion of the skeleton, whether from side to side, 

 from back to belly, from head to tail; and there is no obvious reason 

 why their number should not be constant, or at least as much so as 

 that of the cervical vertebras, since the degrees of mobility required 

 of the latter in different species, are far more numerous and decided 

 than appear to be required from the trunk ; yet no such constancy 



