60 BEITISH FOSSILS. 



central lobe of this organ. Nevertheless, it has the expanded 

 sagittate base usual in this section of the genus, and from this 

 base extends a narrow lanceolate piece, ridged along the centre, 

 and shelving (to pass under the lateral lobes) on the sides. These 

 contract 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. 



PIG. 6. 



Portion of Epis- 

 toma. 



Post-oral Plate (Metastoma), Plate II. fig. 4. This plate is seen 

 in situ (Plate XV. fig. 1, g\ and sometimes occurs of large size, 

 above three inches long. The form is broad-linear above, then 

 contracted at the lower fourth to half the width ; this lower por- 

 tion 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 plicse, which are seen 

 occasionally scattered over ull the plate. The elongated form of 

 this organ, compared with that of the preceding species, corre- 

 sponds 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. punc- 

 tatus, Plates VIII., IX.) 



EndognatJia (Mandibles, &c), Plate XIII. figs. 1-4, Plate XV. 

 fig. 1). These were omitted by accident from Plate II., but are 

 very important for understanding the structure of Pterygotus, since 

 they appear to indicate that the animal possessed two pairs of 



