138 DR. J. MUEIE ON THE FORM AND STEUCTURE OF THE MANATEE. 



and the other reasons adduced point somewhat weightily to the presumption that the 

 third is the deficient cervical vertebra in the Manatee. 



Eegarding dorsal vertebrae, Krauss's tables show 16 and 17 to be most frequent, 

 though Stannius records 16. I select four examples from my own observations in 

 support of inconstancy in dorsal vertebrae and ribs. In the Stuttgart Museum there 

 is a skeleton of a young animal (received from Herr Kappler, 1864) of which the spinal 

 numbers are 6 cervical, 16 dorsal (with ribs), and 25 lumbo-caudal, =47. In the 

 , Zoological Society's juvenile male there obtained 6 cervical, 17 dorsal (with ribs)', and 

 25 lumbo-caudal, =48. In an adolescent skeleton in Heidelberg Anat. Mus. I counted 

 6 cervical, 17 dorsal (17 ribs on left and 16 on right side), lumbo-caudal 25, but possibly 

 two ossicles deficient, =48 or 50 ] In the Zoological Society's female the numbers (with 

 ribs) were, 6 cervical, 18 dorsal, and 27 lumbo-caudal, =51 in all. Excepting the Heidel- 

 berg specimen, the terminal tail-elements were intact, being connected by ligaments. 



The so-called lumbar vertebrae are two or three, according to circumstances. Thus, 

 one thing with the other, it results that the spinal column of the genus presents con- 

 flicting anomalies. 



2. The Spinal Ligaments. 



There is a certain amount of rigidity in the spinal column of the larger-sized 

 Manatus, resultant from the very limited amount of intervertebral substance. In 

 the older female specimen the thickness of the intervertebral cartilage barely exceeded 

 one tenth of an inch, the bones in consequence approximating very closely. This 

 deficiency of the elastic cushion of soft cartilaginous substance is not confined to 

 any one region of the spine, but is met with from the cervical almost to the tip of the 

 caudal vertebrae. In the young animal there is much greater flexibility of the spine ; 

 as, indeed, one would anticipate, seeing this is the universal rule in the Mammalia. 



Counterbalancing the deficiency of intervertebral cartilage, there is an ample develop- 

 ment of ligamentum subflavum in all the spine-bearing vertebrae. In the neck, as might 

 be expected from the antero-posterior compression of the six bones, it is thin, and 

 accommodated to the wide arches. Between the dorsal spinous processes, especially 

 from the fifth backwards, it is remarkably thick and strong. In some interspaces it 

 measured above an inch broad, and almost as much deep. From the true dorsal ver- 

 tebrae backwards it decreases in the ratio of the size of the spines. This ligamentum 

 subflavum is composed chiefly of yellow elastic tissue. 



The equivalent of the anterior common ligament of the vertebral column, here hori- 

 zontally placed, is only of moderate breadth and strength. From the seventh dorsal 

 segment to the lumbar vertebrae the ligament in question is mainly noticeable as 

 existing between the keeled part of the ventral processes. 



' I have to regret an error in Plate XXVI. fig. 37, where eighteen ribs are drawn. I bear due share of blame 

 for not detecting in time that my artist had mistaken a fibrous band coming from the cartilaginous tip of the 

 first lumbar as a rib ; the eighteen costse in the female doubtless also misled us both at the moment. 



