186 ON THE ANATOMY AND AFFINITIES OF 
posterior extremity to the under surface of the hinder half of the 
carapace ; that it lay behind the mouth, and had all the relations of 
a metastoma, and it will be seen by and by that certain existing 
Copepoda possess a metastoma very like it, except in its very great 
proportional size. 
Putting together the different facts furnished by the remains of 
the two species of Ptergoéus, which have been described, it results,— 
1. That the body in this genus was composed of a number of 
segments, which might be thirteen in number. 
2, That these segments exhibit a peculiar ornamentation. 
3. That the terminal segment, or telson, is liable to considerable 
specific variation. 
4. That the anterior segment is larger than the rest, and forms a 
carapace, on whose antero-lateral margin two large oval convex eyes. 
are seated.} 
5. That, attached to the under surface of the carapace, there are 
eight (or ten) distinct organs, two single and median, and three or 
four pairs. 
6. The former are, in front, the great epistoma; behind, the 
metastoma; between these lay the oral aperture. 
7. The latter are, anteriorly, the chelate organs (antenuz) ; pos- 
teriorly, the ectognaths, immediately in front of which lay one or two: 
pairs of endognaths. 
8. That there is no good evidence of the existence of any other 
appendages. 
Almost all the remains certainly assignable to Prerygotus, which 
have passed through my hands, are easily referable to one 
or other of the classes of organs mentioned above, and exhibit 
no anatomical peculiarities worthy of comment, but a _ few 
specimens present difficulties to whose discussion I will now 
proceed. 
1. Plerygotus punctatus——The only representative of the endo- 
gnaths of this species which I have examined (Plate XI. [Plate 22] 
fig. 5) exhibits a structure somewhat different from that of the 
perfect endognath of P. dzlobus and anglicus. The anterior and 
internal angle is rounded off; the series of teeth commencing be- 
hind, and not at the anterior extremity of, the inner edge. The 
straight inner portion of the anterior margin is spinose, and forms a 
considerable angle with the outer portion, which bears the palp. 
' Notwithstanding the peculiar character of the markings upon the corneal surfaces of 
these eyes, I wait for better evidence than I have hitherto met with, before deciding that 
they were really compound, and that these markings indicate corneal facets. 
