BRITISH FOSSILS. 25 



is no independent evidence of the existence of a second antenna in 

 the genus, and that the position of the filiform appendage on the 

 left side may be purely accidental. 



4. The two singular fossils figured in Plates XII. fig. 16, and 

 Plate XIII. fig. 16, are the only remains, which, from their sculpture, 

 may be, with every probability, referred to Pterygotus, but which 

 are not as yet referable to their proper place in the organization. 



These parts present, at first sight, a striking resemblance to the 

 terminal palette of an ectognath, with a portion of the penultimate 

 articulation. They are described at length in the systematic por- 

 tion of this Monograph (p. 97) ; and I will, therefore, only remark 

 in this place that, as the ectognaths of P. problematicus and 

 arcuatus (the species to which these remains probably belong) have 

 not yet been observed, it is possible that they may be their distal 

 joints. Pending sufficient proof that such is the case, however, it 

 must be borne in mind that they differ in several important respects 

 from any ultimate and penultimate joints of an ectognath at present 

 known. The only alternative which suggests itself is that these 

 parts may have been thoracic or abdominal appendages. 



5. The fossil figured in Plate XIII. fig. 17, is evidently crustacean, 

 but it exhibits no character by which it can be identified as a part 

 of a Pterygotus. It is a broad quadrate plate, abruptly truncated 

 on two of its opposite sides, one of which is much longer than the 

 other, and having the other two margins equal and rounded. Two 

 folds or ridges run from one truncated edge to the other, and divide 

 the plate into three lobes, a median and two lateral. The median 

 lobe presents a series of curved lines or ridges disposed symmetrically 

 on either side of the median line. Similar folds in the one lateral 

 lobe are more or less symmetrically arranged with regard to those 

 on the other lobe. 



The shorter truncated margin of the plate presents two 

 impressions, like the remains of articular surfaces, and there are 

 similar impressions at the outer extremities of this margin. Of 

 these, that 011 one side gives attachment to a long jointed appen- 

 dage presenting six distinct articulations. The basal joint is short 

 and much wider than the others. The distal edge of the fourth 

 presents a series of strong spines, the sixth has the form of a curved 

 claw. 



I know of only two crustacean structures with which this body 

 can be compared, the one is a carapace, the other the swimming 

 limb of a copepod with its coalesced, lamellar, basal joints greatly 

 developed (compare, Plate XVI. fig. 7). 



