8 C. F. ROUSSELET ON THE SPECIFIC CHARACTERS 



the jaws of A.hrightwelli (PI. XXXIII., Fig. 4), from his own obser- 

 vation. This figure is fairly correct, but the rami are a little 

 more angular in shape, so that the space they inclose between 

 them when closed is not so perfect an oval as there represented. 

 In Fig. 3 of the accompanying Plate I now give an exact repre- 

 sentation of these jaws as I have invariably found them ; this 

 figure has been carefully drawn from a very fine photograph taken 

 by Mr. M. Poser. It will be seen in particular that there is no 

 tooth near the middle on the inner side, nor a hook-shaped 

 prolongation on the outside at the base of the rami, as repre- 

 sented in Bright well's and Dalrymple's figures ; but near the 

 lower outer angle of the rami there arises a very small blunt 

 projection, which can only be seen from a side view. At the tip 

 there is really but a single point, and it is formed as represented 

 in Fig. 3, c ; on crushing the jaws the thin chitinous ridge seen in 

 side view (Fig. 3, b c) is bent over and simulates a second tooth, 

 A tooth -like point, often seen near the tip of the ramus by 

 focussing up or down, is a false appearance, being the outer edge 

 of the broader part of the apex of the ramus. In a side view the 

 rami are seen to be nearly as deep as they are broad. My view 

 of their structure is that the chitinous material is bent at right 

 angles throughout the length of the rami, forming an inverted "L" 

 in cross section (~]), which of course greatly adds to their strength. 

 The chitin is very thin and the jaws are very light. This structure 

 alone will account for the different appearances they present in 

 front and side views, and will at the same time give the necessary 

 strength these very delicate, but comparatively long, structures 

 evidently possess. Attached to the outer edge of the ramus on 

 each side there is a thin, delicate, curved chitinous rod, w^hich 

 was glimpsed by Dalrymple, and has been called the " secondary 

 or reserve jaw." It arises a little below the middle from a 

 small thin plate, which is fixed to the ramus by three or four 

 delicate rods, one of which is longer, goes upwards, bifurcates, 

 and is attached to two sides of the ramus. These " reserve jaws " 

 remain, and are readily seen when the jaws have been dissolved 

 out with caustic potash ; they open and close and move with the 

 main jaws, but are too weak to be of any use in seizing prey 

 or food. They were considered by Mr. Gosse as early as 1855 to 

 represent the remnants of the mallei, which at first sight appear 

 wholly wanting in all Asplanchnas. In this interpretation Mr. 



