222 DESCRIPTION OF THE SPECIES OF PTERYGOTUS 
near the tip, which is again expanded a little and then forms a 
cordate pointed lobe. The piece is one inch and three-quarters long, 
and half an inch wide at its broadest part (below the middle). I do 
not know to what species it can be referred. 
There is yet a third and very large plate found in the same 
stratum, which, from its shape and structure, appears likely to belong 
to an allied species, and differs from both the above described. A 
woodcut (fig. 6) is here given of it, half the natural length. It is 
keeled along the middle, and rounded near the tip. 
Post-oral Plate (Metastoma), Plate IJ. [Plate 13] fig. 4—This 
plate is seen zw sztu (Plate XV. [Plate 26] fig. 1, g), and sometimes 
occurs of large size, above three inches long. The form is broad- 
Fic. 6. 
Portion of Epistoma. P. acwmdnatus (2?) Lesmahago. 
linear above, then contracted at the lower fourth to half the width ; 
this lower portion also has parallel sides, is rounded below, and 
furnished with a strong central keel, which is probably the line of its 
attachment to the head. The upper end is deeply bilobed, the lobes 
elliptical, and the notch very narrow ; except the raised keel along 
the hinder portion the entire plate is flat, the upper portion marked 
with large and rather obscure semicircular plicze, which are seen 
occasionally scattered over all the plate. The elongated form of 
this organ, compared with that of the preceding species, corresponds 
with the longer shape of the head; and it may, perhaps, guide 
in determining what the form of the carapace of other species was, 
where only the metastoma is preserved (as in P. punctatus, Plates 
VIIL, TX. [Plates 19, 20]). 
Endognaths (Mandibles, &c.), Plate XIII. [Plate 24] figs. 1-4, 
Plate XV. [Plate 26] fig. 1.— These were omitted by accident 
from Plate II. [Plate 13], but are very important for understanding 
the structure of Prerygotus, since they appear to indicate that the 
animal possessed two pairs of masticating organs (exclusive of the 
serrate bases of the swimming feet) as before indicated for P. per- 
orvatus. 
