CERATIOCARIS LEPTODACTYLUS. 15 



pits, which were the bases of setse, on the exposed surface of one of the specimens 

 in the University Museum at Cambridge (PI. VI, fig. 4 6). 



In each case we have only caudal spines to deal with ; but M'Coy's specimens 

 (' Brit. Pal. Foss.,' pi. 1 E, figs. 7, 7 a, 7 b) are much more slender than Murchison's 

 (' Sil. Syst.,' pi. 4, figs. 10 and 64, and ' Siluria,' last edit., pi. 19, figs. 1, 2), and 

 less strongly ribbed ; and therein they have specific differences. 



Some similar caudal appendages (Mus. Pract. Geol. |f) occur in the Lower- 

 Wenlock Rock of Helm Knot, Dent, Yorkshire. Some doubtful fragments of ulti- 

 mate segments of the abdomen are shown in PI. VI, fig. 6, and PI. X, fig. 8. 



In 1860 Mr. Salter apportioned to this species a certain kind of carapace which 

 we think belongs to a new species (G. Halliana, which see further on). 



PI. VI, figs. 4 a, 4 b, 4 c. Cambridge Museum a/924. Described and figured by 

 M'Coy, ' Brit. Pal. Foss.,' 1851, pi. 1 B, figs. 7 and 7 b, as Pterygotus leptodactylus, 

 from Leintwardine. This specimen is in faintly greenish-grey mudstone, slightly 

 calcareous along thin seams at the edge. It consists of a small fragment (smaller than 

 when figured by M'Coy) of the distal (lower) end of an ultimate segment, longitudi- 

 nally striate, and a ridged telson. The latter, not fully exposed at the end, retains 

 portions of its test ; some of these pieces, low down, are small, but the proximal 

 portion or head of this telson is well characterised by its leaf -marked or latticed 

 test (4 a, 4 c). The junction of segment and telson have been disturbed by pressure. 



The outer ridge of the telson and the third ridge inwards bear numerous marks 

 of the bases of small prickles (figs. 4a, 4b, 4c). Whether the outer edge repre- 

 sents the back or the side of the dorsal ridge (that is, if it bore more than the one 

 visible row of marks) is doubtful ; the telson may have had a double row of 

 prickles along its back, one on each side of the ridge. The inner row on the third 

 ridge is an interesting and special feature. 



Figure 5 (Cambridge Museum a/923). Described and figured by M'Coy (' Brit. 

 Pal. Foss.,' pi. 1 E, fig. 7 a). In olive-grey sandstone, not calcareous. From Leint- 

 wardine. A longitudinally ridged telson, badly preserved, but retaining a striated 

 fragment of ferruginous crust. 



PI. VI, figs. 6, 7, 8,9. Museum Practical Geology {^ 1)2)3>4 . Labelled " 0. 

 Murchisoni, M'Coy, Coniston Grit., 1 Helm Knot, Dent, Yorkshire. Collected 

 by Prof. M'K. Hughes." See also ' Catal. Cambr. Silur. Fossils,' 1878, p. 84, and 

 ' Geol. Mag.' vol. iii, p. 205. Hard, olive-green, and brownish mudstone, not cal- 

 careous, with poorly-preserved ferruginous impressions. 



Fig. 6. Probably part of an ultimate segment, longitudinally striated. The 

 obscure, irregular, deeper, and continuous hollow may possibly have reference to a 

 telson. 



Fig. 7. A style (broken) and two stylets. All ridged throughout; the first 

 1 Equivalent to the " Wenlock Shale " (Lower Wenlock). 



