209 



CHAPTER XL* 

 CAMBRO-SILURIAN FOSSILS. 



Cytheropsis Aldensis (M^Coy). 



Sp, Char. Arcuato-oblong, dorsal margin much arched, greatest 

 convexity about the middle, sloping more towards the ante- 

 rior, which is slightly smaller than the posterior end ; posterior 

 end broadly arched, anterior end obtusely pointed ; a concave 

 flattened sinus rather more than half the length of the shell 

 in the ventral margin, rather nearer to the anterior than the 

 posterior end ; an obscure roughened spot slightly nearer to 

 the anterior than the posterior end, and slightly nearer to the 

 dorsal than the ventral margin j valves moderately and evenly 

 gibbous ; surface very minutely punctured under a strong lens. 

 Length 1| millimetre, depth about f rds the length. 



This little species is accompanied by a more elongate, oblong, 

 less arched form of greater rarity, which may either be a distinct 

 species or the male. 



Extremely abundant in the dark earthy Jimestone of Aldens, 

 Ayrshire. 



{Col. University of Cambridge.) 



Harpes parvulus (M'Coy). 



Sp. Char. Cephalic shield semielliptical ; flattened limb very wide 

 (slightly exceeding the length of the glabella in front) with a 

 thick margin ; glabella narrow cylindrical, the slightly nar- 

 rowed front about the width of its base, within the front mar- 

 gin of the cheeks j cheeks convex, one-third wider than the 

 glabella at base ; oculiform tubercles on a line with the ante- 

 rior margin of the glabella, and slightly nearer to it than to 

 the lateral margins ; surface minutely granulated. Length of 

 cephalic shield 3 lines, proportional length of front margin 

 about y^Q^Q, length of glabella -^-^q. 



This little Harpes is interesting as the first occurrence of the 

 genus in Silurian rocks in Britain, although Col. Portlock has 

 described two species which are common in Ireland. The spe- 

 cies is remarkable for its small size and extremely wide marginal 

 plate. 



Limestone of Wrae quarry. Upper Tweed. 



{Col. University of Cambridge.) 



* The species in this chapter date from the * Annals ' for Nov. 1851. 



o 



