PLATANISTA. 529 



stylohyal (sh), which is more or less cylindrical and twice curved on itself and 

 somewhat compressed at its cranial end. In the foetus, the basihyal is a minute 

 ossicle, whereas the thyro and stylohyals are well ossified. 



Vertebral column (PL XXXIX).— The vertebral formula is this: C 7, D 10 

 or 11, L 7 or 8, 26 or 27 = 51 or 52. I have examined twenty perfect skeletons 

 and have never found less than fifty-one vertebrae. 



Characters. — The vertebral column (figs. 3 and 4) describes two curves ; the 

 six anterior cervical vertebrae having a slight upward bend, while the dorsal 

 vertebrae from the last to the seventh have an upward and forward curve, whereas 

 from the latter to the first, the direction is downwards and forwards. The 

 lumbar and caudal vertebrae are nearly in a straight line, the terminal seven verte- 

 brae being slightly curved downwards. The cervical vertebrae are all distinct. 

 The centra of the vertebrae are rather narrow antero-posteriorly, their breadth 

 being considerably in excess of their length, in any portion of the column. The 

 strongly developed spinous processes are directed backwards in the anterior verte- 

 brae, up to the fifth dorsal ; the process of the next vertebra is erect, whilst all the 

 processes of the succeeding vertebrae are curved forwards in a marked degree, and 

 the processes entirely disappear in the fortieth segment. The neural canal, which 

 is capacious in the cervical and first part of the dorsal region, being much broader 

 than high, rapidly diminishes in capacity, its breadth in the last dorsal vertebra 

 being one-half of what it is in the first rib-bearing segment. In the lumbar region 

 the height gradually exceeds the breadth. In the tenth caudal it is reduced to 

 a tube about one line and a half in diameter. The strongly- developed metapophyses 

 first appear in the fifth dorsal, and can be traced backwards as far as the fourteenth 

 caudal, and are another marked feature of this dolphin. The last trace of zygapo- 

 physes occurs on the sixth lumbar vertebra. Transverse processes are present 

 from the atlas to the fifteenth caudal in a more or less decided manner, but they 

 are especially strongly developed in the lumbar and first five or six caudal vertebrae. 

 Special characters of individual vertebrce. — The atlas is much broader than 

 high, and has a strong transverse process, which is directed backwards and outr 

 wards. It springs from the pedicle external to the outer border of the posterior 

 zygapophysis, and from between it and the anterior zygapophysis, i.e., above what 

 would be the position of the neurocentral suture; it would thus appear to be 

 serially homologous with the superior transverse process of the vertebrae immediately 

 behind it. The inferior transverse process is most intensely developed in the sixth 

 cervical vertebra, where it is directed forwards and outwards, and it can be distinctly 

 traced forwards to the fourth cervical, here springing directly from the side of the 

 centrum, but its superior root originating from the inferior root of the superior 

 process. As there can be but little doubt that the transverse process of the axis is 

 serially homologous with the superior transverse process behind it, the process of 

 the atlas which occupies a relatively higher point on the neural arch, and which 

 follows the direction common to the two superior transverse processes behind it, 

 is also the homologue of these processes. The neural arch is lower and more 



T 3 



