22 ON THE STRUCTURE OF TRACHYPTERUS ARCTICUS, 
3. The lateral ridges usually formed by the pterotics, epiotics, and parietals are 
slight or absent. 
4. One element only is present in place of the prootic and opisthotic ; it agrees 
with the bone named opisthotic by T. J. Parker, but is rather to be identified with 
the prootic. 
5. The sphenotics send descending processes to meet the parasphenoid, and 
form post-orbital pillars as in Regalecus. 
6. The ex-occipitals are united below, excluding the basi-occipital from the 
foramen magnum, though not from the floor of the brain-case. 
7. The orbitosphenoids and alisphenoids are large bones, and meet in the 
middle line ventrally as in Regalecus. The basi-sphenoid, absent in Regalecus, is 
here well developed. 
8. The roof of the skull is largely composed of cartilage, by which the frontals 
are separated from one another, and a groove is formed in which processes of the 
pre-maxillee work back and forward. 
9. The parietals are small. They do not appear on the outer surface of the 
skull, being covered over by the post-temporal; nor do they appear on the inner 
surface of the skull-wall, being shut out by the frontal passing in to articulate with 
the supra-oceipital. 
10. The mesethmoid is very large. 
11. The vomer clasps the pre-nasal cartilage and meets the parethmoids. 
12. The premaxille, the vomer, and the dentaries bear teeth. 
13. The nasal processes of the premaxillae and the general mechanism for the 
retraction and protrusion of the jaw is similar to that in Regalecus. The mandible 
is short and high, but is much larger relatively than in Regalecus. 
14. The nasals are loosely attached, and are very small indeed, smaller even 
than in Regalecus. 
15. There are two pre-orbital bones, of which the upper and anterior bounds 
the nostril below, as in Regalecus. 
16. The branchial arches bear denticulated gill-rakers, smaller and more simply 
arranged than in Regalecus. 
17. The copule or medio-ventral elements of the branchial skeleton are at least 
nine in number. 
18. The vertebral column consists of 101 vertebrze, whose neural arches support 
about 173 interspinous bones :—a greater number of vertebree, and a less number of 
interspinous bones than in Regalecus. 
19. Ribs are absent or very minute. 
20. The post-temporal, as in Regalecus, is simple and not forked. 
