1887.] NATURAL SCIENCES OF PHILADELPHIA. 353 



back out of their usual place, over about thirteen segments beyond 

 the occiput, and the translocation of the pectoral of Ophiognathus 

 is scarcely less. In both these cases the translocation is due to the 

 extension backward of certain portions of the inferior arches of the 

 skull, and of the mouth, as a result of which, the brt^nchial appara- 

 tus and lieart have also been displaced and lost their attachment or 

 contiguity to the skull, while the carotid arteries have been length- 

 ened to an extent which is altogether wdthout a parallel amongst 

 fishes. 



Amongst median fins the anterior dorsal of Lojyhius is known 

 to undergo considerable displacement forward, according to the 

 published figures of A. Agassiz, showing the development of this 

 form. Even in the history of the metamorphosis of the tail in het- 

 erocercal types, I have attempted to show (Origin of Heterocercy) 

 that associated with the deflection upwards of the end of the cau- 

 dal axis, due to a definite combination of mechanical conditions, 

 there has also occurred a translocation forwards, crowding together 

 of the inferior basal and hsemal supports of the caudal rays. 



Turning again to the evidence, purely morphological and embry- 

 ological, we find that the last pair of spinal nerves in Phoccena make 

 their exit at the 45th vertebra, in Man at the 27th, in Lepus at the 

 29th, which facts when contrasted, sufiiciently prove that there have 

 been changes in the position of the source of the innervation of the 

 limbs. The additional evidence which we possess showing that the 

 hind limbs of the Cetacea and Sirenia have sufl^ered displacement 

 and may not be exactly homologous with the hind limbs of other 

 vertebrates, consists in the fact that the hinder limb-buds (future 

 flukes) grow out at the sides of the cylindrical tail and but slightly 

 in advance of its Lerminatiou, as low horizontal folds which have 

 degenerated into the flukes, which now consist internally of the low- 

 est grade of tissue, viz : fibrous connective, covered by the closely 

 adherent integument. 



In violent contrast to this we have the mode of development of 

 the limbs of the Physoclisti at a parallel stage as buds or outgrowths 

 immediately behind and with scarcely an interval between them- 

 selves and the rudiments of the pectorals immediately in advance of 

 them. As a result of this, the paired spinal nerves which innervate 

 the pectoral and pelvic limbs, form a continuous series with no in- 

 terval whatever between them as shown by the accompanying ta- 

 ble, while the greatest interval between the last brachial and first 



