14 ON THE STRUCTURE OF TRACHYPTERUS ARCTICUS, 
much larger and wedge-shaped, being about one-eighth of an inch in length above and 
one-twelfth below. The third centrum is also wedge-shaped, but slightly: below, it 
measures three-sixteenths of an inch; and above, one-eighth (it appears more 
wedge-shaped in one of our specimens than in the other). From the third to the 
ninety-second there is a gradual increase in length of centrum; and the applied 
faces are now parallel. The fourth is very nearly one-fourth of an inch in length ; 
the fifth is slightly more than one-fourth of an inch; the eighth is nearly 
five-sixteenths; the seventeenth is over five-sixteenths; the thirty-ninth is 
three-eighths ; the seventy-eighth is five-eighths ; and the ninety-second is thirteen- 
sixteenths. Beyond the ninety-second there is a gradual decrease. The ninety- 
seventh is nine-sixteenths; the ninety-eighth, seven-sixteenths ; the ninety-ninth, 
nearly three-eighths; the one-hundredth, nearly five-sixteenths ; and the last one, 
three-sixteenths of an inch, not including the terminal cartilage. 
Ribs are either altogether absent, or are so very minute as to have escaped our 
notice. 
The first vertebra, in spite of the thin discoidal centrum already referred to, is 
in respect of its neural arch and spine the strongest of the series. The centrum is 
firmly attached by ligament on the one hand to the occipital condyle, and on the 
other to the second vertebra. It is opisthoccelus, and presents near the middle of 
the rounded disc the foramen found in all. The centrum is oval, with the short 
diameter vertical, and half as long as the longer diameter. The arch is flattened 
anterior-posteriorly. The spine is club-shaped ; there are no articular nor transverse 
processes, though the latter are, by a rare anomaly, present in Regalecus. 
The second vertebra has a spine nearly as strong as that of the first. The 
wedge-shaped centrum is amphiccelous, but the cup is much shallower in front than 
behind; it is in section oval rather than circular, and has the typical structure 
otherwise of two opposed cones with strengthening lamelle. The posterior 
zygapophyses are slightly developed, the anterior are still absent. 
In the third vertebra the anterior zygapophysis is developed, and the posterior 
is now large and typical. The spine in this and in the succeeding vertebra is still 
flattened and club-shaped, but less strong than in the first and second. 
The transverse process is present for the first time in the fourth vertebra. On 
the ventral surface, between the two transverse processes, are in one of our 
specimens four, in the other eight, of the longitudinal lamellz ; the eight in the one 
case showing signs of origin from the splitting of an original four. 
The spine in one specimen is club-shaped like the preceding ones, but in the 
other it is almost reduced to the slender rod found in the succeeding vertebrae. It 
ought to be mentioned here that the neural arch of .this vertebra, along with the 
preceding ones, bridges two fossze, and not one as in the typical vertebrae. The 
median ridge which divides these fossee gets gradually absorbed in the next one or 
two succeeding’ vertebree. 
