﻿66 BRITISH FOSSIL CRUSTACEA. 



PL XIII, where the anterior thoracic segments are displaced, yet its exact contour 

 cannot very well be represented. 



The anterior body-segments are much arched forward in the centre ; the lateral area is 

 recurved as in the other species ; and the anterior border appears to have projected under and 

 formed an articulation with the preceding segment (see PI. XV, fig. 1, segments 9 and 10). 



The sculpture of the body-rings (PI. XIII, fig. 1 g) extends over less than half their 

 surface. 



The plicae are open forwards, very small, often almost linear on the front margin, 

 and the remainder are less than semicircles. The anterior border of the segments is 

 rounded off and smooth. A transverse faintly impressed line separates the anterior 

 sculptured half from the posterior smooth portion ; but this is not always seen. The 

 lateral borders of the segments are not crenated. 



The telson (PL XV, fig. 2, 20), is bilobed in form like the preceding varieties a and |3 ; 

 and the penultimate segment has its anterior border contracted, its sides curved, and its 

 posterior angles produced in a similar manner. 



On the Branchice in Pterygotus. — The determination by Dr. James Hall, in America, 

 of the true position of the thoracic plate or operculum in PJurypterus, 1 having since been 

 fully confirmed with regard to the British species of Pterygotus? Slimonia? and Bury- 

 pterus? and also its homology with the operculum in Limulus (PL IX, fig. 1, 1 a) — 

 beneath which are placed the respiratory organs — it was reasonable to expect to find 

 evidence of branchiae in Pterygotus also, considering the wonderful state of perfection 

 in which many of the remains of this genus have been preserved. 



But it was not until 18G7 that I obtained satisfactory evidence of their existence. 

 I first drew attention to them in my ' Third Report to the British Association on the 

 Structure and Classification of the Possil Crustacea,' at Dundee, in September of that 

 year, and I have since published figures of some detached leaflets in a paper read before 

 the Geological Society in March last. 5 



I first detected them associated with the specimen figured in PL XII, figs. 1 a (dr) 

 and 1 d; next in that in PL XIII, figs. 1 a and 1 h. When at Dundee (in 1867), 

 I obtained from Mr. Slimon a portion of shale having several detached leaf-like organs 

 preserved upon it, 6 corresponding in form and surface-markings with those referred to 

 above, and which occur with specimens of entire Pterygoti and lying in such a position 

 upon the slabs as to leave little doubt that their normal place of attachment would be 

 under the thoracic plate or operculum, as in the recent Limulus. 



1 Hall, 1859, 'Geol. Surv. New York,' " Paleontology," vol. iii, pp. 392—413. 



2 H. Woodward, 1867, « Quart. Journ. Geol. Soc.,' vol. xxiii, pp. 28—37, pis. 1 and 2. 



3 Ditto, ' Intellectual Observer,' 1863, vol. iv, pp. 229—237. 



4 Ditto, 'Geol. Mag.,' 186-J, vol. i, pp. 107—111, pi. v, fig. 8. 



5 ' Quart. Journ. Geol. Soc.,' 1868, vol. xxiv, p. 294, pi. x, figs. 3 a, 3 h. 



6 The same which are figured in the ' Quart. Journ.' above referred to. 



