114 BRITISH FOSSIL CRUSTACEA. 



narrow and has a raised keel down the centre, two inches long, which bifurcates at the 

 hinder border. Near the centre of the widest part are two subcentral parallel ridges 

 about 1^ inches long. A few scale-like markings are seen near the upper notched end 

 of the lip-plate. This plate was, no doubt, attached along the line of the posterior median 

 ridge and by its hinder border (p). It overlapped the inner borders of the great basal 

 ectognaths, and served as a mentum, or lower lip, being attached behind, not in front 

 of the buccal orifice, as is the hypostome in the Trilobita. 



The Thoracic Plate or Operculum (PI. XVII, fig. 1, 7, c, and fig. 2 ; and PL XX, figs. 

 2, 3, & 4). — The displacement of this plate in the description of this species by Mr. Salter 

 ( f Geol. Surv. Mem./ Mon. I, p. 59), where, as in the descriptions of Plerygotus, it is 

 called " the conjoined epistoma and labrum," has already been referred to under Ptery- 

 gotus. (See restoration after Mr. Salter, Part I of this Monograph, p. 27 and p. 39.) 

 It is only necessary to point out that fig. 5, p. 59, of Mr. Salter's description (op. cit.) 

 is identical in form with the plate seen in situ on our PI. XVII, fig. 1, 7, c; and that 

 figured on p. 60, fig. 6 (op. cit.) is the tricuspid median appendage of a plate identical in 

 form with that figured on our PI. XVII, fig. 2, and is known to occupy the same rela- 

 tive position in the body of Slimonia as that seen in situ (7, c) on the same plate in fig. 1. 

 It is, in fact, homologous with the operculum of Zimulus (Part I, PI. IX, fig. 1, 8, and 

 fig. 1, c), and is composed, as in Zimulus, of the modified appendages belonging to the 

 first thoracic somite, which in both Zimulus and Slimonia is coalesced with the cephalon. 



The thoracic plate seen in situ in fig. 1, PI. XVII, measures five inches from border 

 to border, and is 1^ inches in length ; it is divided into two lateral oblong pieces by a 

 median lobe (c), which is 2^ inches long and projects considerably below the rest of the 

 lower free border of the plate. This median lobe in fig. 1 is broadest at its lower 

 rounded extremity, where it is eight lines wide, contracting to six lines where it unites 

 with the two oblong lateral pieces of the operculum ; at its upper end it forms a small 

 triangle, four lines long and four broad, from the apex of which a narrow double line 

 extends to the upper attached border. Two diagonal lines take their rise from the upper 

 triangular end of the median lobe (c), uniting with the upper border of the operculum one 

 inch on either side of its central line. 



The specimen figured on PI. XVII, fig. 2, measures eight inches in breadth and If 

 inches in length ; the median lobe is 3^ inches long ; its termination (c), instead of being 

 rounded, as in fig. 1, is tricuspid, measuring one inch across the side cusps ; the centre of 

 the median lobe is ridged ; the margins are slightly curved, and unite in a lanceolate 

 termination half an inch from the anterior attached border. Instead of the two diagonal 

 lines seen in fig. 1 , three lateral lines are given off from near the lanceolate upper end of 

 the median lobe, which, expanding on either side, reunite with each other, and so with 

 the upper and inner attached border of the operculum one inch on either side the median 

 line. A slight double border two lines broad surrounds the free margin of the lateral 

 lobes of the operculum. 



