4 



BRITISH FOSSILS. 



Explanation of Plate III. 



Fig. 1. Old individual from Barr, in the possession of Miss Jukes, of Birmingham. 

 Fig. 1*. Side view of the same, showing the shortened foremost pleurae (a), and the 

 inferior eyelid (b). 



Fig. 2. Side view from a fine coiled specimen in the cabinet of John Gray, Esq., 

 Dudley. 



Figs. 3, 3*. Two views of a young one ; in the cabinet of Augustus Lewis, Esq. 



Plate IV. 



Fig. 1. The dissected figure, a, head; b b, cheeks ; c, thorax segments ; d, tail. 

 Fig. 2. Portion of surface above the eye, showing the wavy lines and puncta. 

 Fig. 3. Ditto, with puncta larger than usual. 

 Fig. 4, Magnified pleurse, with lines and puncta. 



Fig. 5. Part of tail, the puncta only present over the hinder portion of it. 

 Fig. 6. Pleura magnified, showing the fulcrum a, the backward bend at b, the sharpened 

 forward edge c. 



Fig. 7. Large head from Littlehope. 



Fig. 7*. The same, lower side, a, rostral shield ; b, rostral suture (Burmeister). 



Fig. 8. Shows the thickened portion of shell just above the eyelid at a ; impression of it 

 on the stone beneath at b. (In this specimen it is destitute of puncta.) 



Fig. 9. Side view of part of tail ; at a, the fulcral point ; b, the articulating surface, 

 with lines, but not puncta. 



Fig. 10. Incurved portion of tail ; the inward surface with elevated granulate lines. 



Fig. 11. A ditto, magnified. 



The specimens figured in this plate are in the Geological Survey Collection. 



The section Bimastus is related to the more typical species of IIJcbuus, just as Nileus, 

 of Dalman, is to his Asaphus palpebrosus. The distinct trilobation has vanished, and the 

 axis is so wide as to reach the fulcral point, and be nearly coincident with it. There is 

 a slight backward bend of each pleura just before the fulcrum, which serves to steady the 

 articulation of the piece behind by overhanging it a little, while this piece, in turn, over- 

 hangs the preceding at the fulcrum, precisely as in the articulation of the abdomen of the 

 lobster and cray-fish. Beyond the fulcral point the pleura is always more or less sharp- 

 ened, to pass under the preceding joint in the act of bending ; and, as in this operation, 

 the back is lengthened, and the soft parts of the axis would be exposed by the separation 

 of the segments, a convex articular portion is added on the forward edge of each segment, 

 which fills the gap, and retires beneath the preceding joint, when the animal is extended. 

 This is the structure of all trilobites which have been observed to bend or roll, and it is 

 similar to that met with in recent Crustacea. 



The subgenus is known in the Lower Silurian rocks of America, and the Upper Silurian 

 of Britain. 



J. W. Salter, 



June, 1849. 



