THE GENUS PTERYGOTUS 185 
‘or second in order, it will be better to term an oral limb in front of 
the ectognath, an “endognath.” The specific name of ‘“ mandible” 
must be assigned to whichever of these endognaths further research 
proves to be anterior. 
The position and general nature of two of the three pairs of 
appendages of Pverygotus having thus been determined, the third 
presents no difficulty. In the specimens of P. dz/obus, represented in 
Plate I. [Plate 12] figs. 1, 3, indeed, the pair of long anterior ap- 
pendages are so much distorted and damaged that their structure 
cannot be satisfactorily made out. There is abundant evidence, 
however, to show that they were similar to the detached chelz 
figured in Plate I. [Plate 12] fig. 6, and these are obviously identical 
with the pincer-like appendage (a) of P. anglicus. 
The epistoma has already been recognised in the plate lying in 
front of the head, in Plate I. [Plate 12] fig. 1; it remains only to 
‘discover the representative of the metastoma, and the position which 
it occupied in the organism. The form and sculpture of the part 
represented in Plate I. [Plate 12] figs. 10, 10 a, testify that it 
answers to the organ sought, and a careful examination of most well 
preserved carapaces of P. dz/obus, shows a corresponding plate, or its 
impression, 7z seé¢. 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 over- 
lapped 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 Prerygotus, as in other Cristacea, 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. [Plate 12], 
remains in its own proper position. 
I have little doubt, therefore, that this plate was attached by its 
