BRITISH FOSSILS. 21 



5. That, attached to the under surface of the carapace, there are 

 eight (or ten) distinct organs, two single and .median, and three or 

 four pairs. 



6. The former are, in front, the great emstoma ; behind, the 

 metastoma ; between these lay the oral aperture. 



7. The latter are, anteriorly, the ch elate organs (antennae) ; 

 posteriorly, the ectognaths, immediately in front of which lay one 

 or two pairs of endognaths. 



8. That there is no good evidence of the existence of any other 

 appendages. 



Almost all the remains certainly assignable to Pterygotus, which 

 have passed through my hands, are easily referable to one or other 

 of the classes of organs mentioned above, and exhibit no anatomical 

 peculiarities worthy of comment, but a few specimens present diffi- 

 culties to whose discussion I will now proceed. 



1. Pterygotus punctatus. The only representative of the 

 endognaths of this species which I have examined (Plate XI. fig. 5) 

 exhibits a structure somewhat different from that of the perfect 

 endognath of P. bilobus and anglicus. The anterior and internal 

 angle is rounded off ; the series of teeth commencing behind, and 

 not at the anterior extremity of, the inner edge. The straight 

 inner portion of the anterior margin is spinose, and forms a con- 

 siderable angle with the outer portion, which bears the palp. 

 The latter is four-jointed, the proximal articulation being large, 

 quadrate and setose along its anterior margin, and provided with 

 a large and strong curved spine towards the outer extremity of that 

 margin. The next joint is elongated and curved, its posterior 

 margin being slightly convex, the anterior similarly concave, and 

 towards the distal extremity bearing a curved spine, like that in 

 the preceding joint. 



The third joint and the fourth are much shorter. The former 

 carries a spine on its anterior edge. The latter has two spines, 

 one anterior and one terminal. Is this the homologue of that 

 endognath of Pterygotus anglicus represented in Plate VII. fig. 4, 

 or does it, as the structure of its basal joint would seem to indicate 

 correspond with the apparently different part shown in fig. 7 ? 



2. Pterygotus acuminatus. On the same slab with a large por- 

 tion of the body and one of the ectognaths of this species, is seen the 

 remains of an appendage whose basal joint clearly resembles the 

 corresponding part in P. anglicus (Plate XIII. figs. 2, 3.) It con- 

 sists of a broad, flattened, quadrate plate, having a series of curved 

 and striated teeth articulated with its well-defined, nearly straight, 



