SKELETON OF EEGALECUS ARGENTEUS. 23 



predecessor, which is itself three quarters of an inch longer than the antepenultimate 

 (91st), the latter being again one quarter of an inch longer than the ninetieth. 



The tail is thus seen to be perfectly diphycercal ; there is not the slightest upturning 

 of the end of the notochord, nor any trace of hypurals. 



The transverse vertical plane passing through the centre of a vertebra corresponds 

 with the septum between two myotomes, or, what comes to the same thing, to the dividing 

 plane between two mesoblastic somites or protovertebree ; so that the demivertebra and 

 the posterior half of the ninety-second vertebra of Regalecus, must be together formed 

 from the last mesoblastic somite. One would like to know whether demivertebrae are 

 of anything like general occurrence, or whether, as a rule, nothing corresponding to a 

 vertebral body is formed in the posterior half of the last mesoblastic somite. 



It must be borne in mind, however, that according to Lutken there is very possibly 

 a caudal fin in young individuals, the condition of the tail in adult specimens being due 

 to mutilation. If this were the case, the demivertebra would be at once explained as 

 the anterior half of a vertebra which had been broken through the middle, as in the 

 mutilated tail of a Lizard. But the somewhat peculiar, i. e. not regularly conical, form 

 of the cavity of the demivertebra, and the absence of any trace of neural or hsemal 

 processes above and below the bony thickening, which may be taken as marking the 

 point corresponding with the middle of an ordinary vertebra, seem decidedly to tell 

 against Liitken's view. A further objection is furnished by the disposition of the 

 posterior interspinous bones (vide infra). 



The Interspinous Bones and Dermal Fin-rays. 



The interspinous bones are for the most part delicate bones (fig . 25, i. sjp. & fig. 5) having 

 the form of a Y with short stout arms and an extremely long slender stem ; in the 

 anterior interspinous bones, the arms occur more or less united into a single triangular 

 plate (fig. 20). In length they agree pretty nearly with the neural spine, varying 

 with the latter in different parts of the body; the majority of them are a little over 

 4 inches long, those at the anterior and posterior ends of the body gradually diminishing 

 to 1-^ inch or less. The arms and upper portion of the stem are fully \ inch in 

 diameter, but the stem narrows below to about -^ inch, that is about the same width 

 as the neural arches. Their dorsal portions exhibit an evident cartilaginous ground-work. 



In correspondence with the length of the vertebrae, the distance between the arms 

 of the spinous bones varies in different parts of the body ; in the middle this distance 

 is about f inch, in the posterior part it rises to fully an inch (fig. 25), and anteriorly 

 sinks to J inch (fig. 20). 



There is also considerable variation in the precise position of the interspinous bones 

 with regard to the neural spines (see fig. 5). In the anterior part of the body there 

 are, as a rule, two spinous bones to each vertebra ; in some cases they alternately lie 

 in an interneural space and immediately over a neural spine, being connected to it by 



