110 Frederick Cliapman : 



Xiphidiocaris falcata, sp. nov. 

 (PI. XXVTII., Figs. 7, la-d). 



Description. — Telson slender, incurved, widely sickle-shaped, 

 gradually tapering from the broad, flattened proximal end to the 

 bluntly-pointed distal extremity. Edges nearly smooth ; a few 

 remnants of small spines present on the concave or inner 

 border. Surface (side) bearing a subcentral ridge, at first flat 

 at proximal end, contracting to a strong ridge passing obliquely 

 toAvards the outer border, where it persists to the apex. So far 

 as can be seen, the surface was relieved by an imbricated, scaly 

 ornament near the base, whilst the distal surface bore series 

 of pittings parallel with the inner side. Surface also marked 

 with fine longitudinal striae, especially on the outer side of the 

 ridge of the convex border, where they become oblique. In 

 cross section the form would be subrhomboidal, with flattened 

 sides and broad grooves along the outer and inner borders. 



Dimensions. — Length of telson, 58 mm. ; width at proximal 

 end, 9 mm. ; width at 20 mm. from base, 5 mm. ; width at 

 50 mmi. from base, 2.25 mm. 



Occurrence. — Silurian (Melbournian). In the blue mudstone 

 of South Yarra. Coll. F. P. Spry. 



Remarks. — The originally described examples of this genus 

 were found in the Lower Ludlow of Shropshire. The only 

 species hitherto known is X. ensis, Salter sp.,i a form distin- 

 guished from ours by the wider curvature of the telson, the 

 more regularly ovate section, and the conspicuous and regular 

 serration of the edges. The surface pits visible in the English 

 examples are interpreted by Messrs. Jones and Woodward as the 

 bases of spines ; a character also seen in the telsons of Cera- 

 tiocaris, to which genus this imperfectly understood form seems 

 allied. 



1 See Xiphocarin ensia, Salter sp. .Tones and Woodward, Mon. Hrit. I'al. Ph.\ llopoda, 

 Pal. Soc, 1888, p. 62, pi. v., figs. 7a-d. 



