486 PROCEEDINGS OP THE GEOLOGICAL SOCIETY. [June 7, 



Hitherto we have been enabled to decide them, each to our own 

 satisfaction, by the two forms of thoracic plates which occnr in the 

 same species ; but if we are to be guided by more general characters 

 than the sexual plate, we must expect the antennse to be modified in 

 the male as in the recent Linmhis ; in which case the two forms of 

 plates in Slimo7iia acuminata indicate two species of females, and 

 the two forms in Pterygotus hUohiis ought to indicate the two species 

 of males with their chelate antennse. 



But, to establish this point, most palaeontologists would desire evi- 

 dence as conclusive as that in the case of Stigmaria and SigiUaria. 



It is interesting to notice evidence of a third species, S. ensiformis, 

 from the Old Eed Sandstone of Forfar (see 'Geol. Mag.' vol. i. p. 198), 

 and a fourth, 8. Symondsii (Euryj)feri(s of Salter), from the Old Eed 

 Sandstone of Herefordshire. Prom an examination of E. megahps, 

 Salter, from Ludlow, I am led to believe that this is also a Stylonurus 

 (see ' Quart. Journ. Geol. Soc' 1859, vol. xv. pi. 10. fig. 1). 



We have thus, with the foregoing and S. Logani from Lanarkshire, 

 probably six species of this curious genus. 



EXPLAJSTATION OF PLATE XIII. 



(Illustrative of New Devo7iian Eurypterida.) 

 Eig. 1. Stylonurus Powriei,V age. Old Eed Sandstone, Foi-far. One-fourth the 

 natural size. 

 2. Scoticus, H. W. Old Eed Sandstone, Forfar. One-sixth the na- 

 tural size. 

 3. „ Old Eed Sandstone, Forfar. One-fourth the na- 

 tural size. 



4. (Eurypterus) Symondsii^. Old Eed Sandstone, Herefordshire. 



One-half the natiural size. 



4. On tlie Discovery of a ISTeav (jenus of Cirripedia in the "Wenlock 



Limestone and Shale of Dudley. By Henry "Woodward, F.G.S., 



P.Z.S. (of the British Museum). 



(Plate XIV. figs. 1-6.) 

 The genus Chiton of Linnaeus (established in 1758) is remarkable 

 among the Mollusca from the aberrant form of its shelly covering. 



Dr. Woodward, in his ' Manual of the Mollusca ' (p. 156), thus 

 describes it : — 



" The shell is composed of eight transverse imbricating plates, 

 lodged in a coriaceous mantle which forms an expanded margin 

 around the body. 



" The first seven plates have posterior apices, the eighth has its 

 apex nearly in front. 



'' The six middle plates are each divided by lines of sculpturing 

 into a dorsal and two lateral areas. 



" All are inserted into the mantle of the animal by processes 

 (apophyses) from their front margins." 



I may add that these plates are always unilinear, and that the two 

 sides are symmetrical, such an instance as a Chiton with un- 

 » See Quart. Journ. Geol. Soc. 1859, vol. xt. p. 230, pi. 10. fig. 1. 



