20 BRITISH FOSSILS. 



P. Ulobus, shows a corresponding plate, or its impression, in situ. 

 When undisturbed, in fact, the metastoma occupies the middle of 

 the under surface of the carapace, the cutting edges of the basal 

 joints of the gnathites overlapping, or being overlapped by, its lateral 

 margins, and its emarginated extremity being turned towards the 

 anterior end of the head, which it nearly reaches. The posterior 

 rounded margin is in contact with the posterior boundary of the 

 carapace. 



A part, having this form and occupying this position, might be 

 regarded as a labrum or as a metastoma. My reasons for giving it 

 the latter appellation are the following : It may be regarded as 

 pretty certain that in Pterygotus, as in other Crustacea, a sculptured 

 surface was free and uncovered by other parts, in which case the plate 

 could not have been attached to the inner surface of the epistoma 

 by its sculptured anterior and outer surface ; nor, for the same 

 reason, could it have been attached by its inner surface to the 

 sculptured outer surface of the epistoma. The anterior margin of 

 the plate reaches so far forwards that it could not have been 

 attached by that margin to the posterior margin of the epistoma ; 

 and. furthermore, when the latter is detached or thrown out of 

 place the metastoma is never found connected with it, but, as in 

 fig. 1 , Plate I., remains in its own proper position. 



I have little doubt, therefore, that this plate was attached by its 

 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-bye 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 Pterygotus, which have been described, it 

 results, 



1. That the body in this genus was composed of a number 01 

 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.* 



* Notwithstanding the peculiar character of the markings upon the corneal surfaces of 

 these eyes, 1 wait for better evidence than I have hitherto met with, before deciding that 

 they were really compound, and that these markings indicate corneal facets. 



