42 Description of Genera and Species. 



flattened like those found in shale. The specimen shows that the animal, as might have 

 been expected, was naturally applanated, for it is nearly always found back up. This 

 specimen also helps to show not only the variation in the proportions of the individuals 

 that have been included in the species, as the individual is very broad for its length, 

 but also that it is accompanied in this case by a duplication of the median longitudinal 

 keel upon the tail segments. This seems to have come about rather by the suppression 

 of the central ridge keel and the accentuation of two lines of the small buttresses that 

 supplanted the transverse band behind the rolling facet which strengthens the anterior 

 part of each of the tergites. In this specimen the lateral keels are continued down the 

 tail as two lines of backwardly-directed spines. Another part shown in this figure is the 

 peak in the mid-line of the posterior margin of the carapace suggestive of a rudi- 

 mentary spine in continuation of the ridge keel. All these points are strongly 

 suggestive of affinities with Tealliocaris and Gnathophausia. Fig. 4d, of the original 

 description of the species, has also the double ridge^ on the tail, but its proportions are 

 much more slender than the form shown in fig. 6, although it was a larger individual. 

 Locality. — All the specimens of this species have been obtained from the celebrated 

 scorpion bed, exposed in the Eiver Esk at Glencartholm, near Langliolm, 

 Dumfriesshire. 

 Horizon. — The exact horizon of this bed is above the " Fell Sandstones," which are 

 supposed to 1)6 represented in the Forth basin by the Granton and Craigleith 

 Sandstones. The scorpion bed, according to this correlation, would therefore 

 be somewhere about the horizon of the Wardie Shales, near the base of the 

 Oil-shale bearing Group of the Calciferous Sandstone Series of Scotland. 

 Collectors. — Mr. A. Macconochie, and a few hy the late Mr. T. Stock. 



Perimecturus stocki sp. nov. PI. Yl., fig. 1. 



Specific Characters.— Ijav'^e form with longitudinally keeled tail tapering gently to 

 source of the uropods. Outer branch of uropods with enormously developed blade-like 

 external keeled part extending beyond the tip of the telson. and furnished with a row 

 of articulated spines on outer margin. 



General Descriptuni. — The integument is thin, smooth and shining, and not much 

 calcified, though evidenth' strong. Only a portion of the trunk is preserved, so that the 

 eyes, the antennular and antennary systems are unknown. The carapace is also frag- 

 mentary, none of its borders have been preserved, and there is no means of telling . 



' Traii.s. Roy. Soc. Eilin., 1882, vol. x.xx., pi. ix., fig. 4d. 



