DITHYROCARIS GRANULATA. 143 



with the upper edge of which the postero-dorsal curvature makes a strong- 

 angular notch. 



The outer coating of the test on the ventral border has a delicately serrated 

 margin ou its posterior half or two-thirds. This is formed of oblique strige (like 

 minute closely-set spines), pointing backwards; but this fringe becomes narrow 

 and dies out on the anterior part of the margin. 



The dorsal edge has on its posterior two-thirds a narrow ridge of small angular 

 rugce, pointing backwards ; and, in PI. XX, fig. 2 a (magnified), this is seen to have 

 a thin and narrow flat flange on each side ; altogether constituting a narrow slip 

 of test, seemingly at first sight intermediate to the two valves,' but really over- 

 lapping them at their junction ; seen also in PI. XX, figs. 1, 2, and 3. 



On the surface the dorsal region of the valve is minutely punctated and 

 bestrewn with minute tubercles, which are coarser in the antero-dorsal region ; 

 and, continued round the front of the valve, they there pass downwards and 

 backwards for a little way in the autero-veutral region, as closely-set, parallel, 

 oblique striae. The ventral moiety of the valve is otherwise smooth. 



A mesolateral rugose ridge, thinning away at its ends, passes along two- 

 thirds of the surface of the valve, between the dorsal and ventral regions. It is 

 composed of overlapping chevron-shaped flakes or scales, making transverse 

 scale-like markings (see PI. XX, fig. 2b). 



This specimen was described and figured in the ' Geol. Mag.,' 1874, p. 108, 

 pi. V, fig. 2. 



In the anterior part of the valve are two small rugose ridges ; one (the 

 "nuchal" ridge about 3 mm. long) near the front end of the dorsal edge, and 

 parallel to it, just where the dorsal ridge ends, is thin and somewhat sinuous ; the 

 other (the " cephalic " or " gastric " ridge about 8"5 mm. long) between the 

 nuchal and the front end of the mesolateral ridge, is more or less sigmoidal, ter- 

 minating behind in a circular turn, which is either solid, or forms a small pit like 

 an ocular spot. (The latter ridge is more persistent than the nuchal indge, which 

 is often obsolete or evanescent on one or the other valve in Dithyrocaris). All 

 the ridges consist of apparently overlapping scales or flattened chevrons, with 

 their angles pointing backwards. These are evidently essential elements in the 

 leaf-ornament or lattice-work on the abdominal segments of Geratiocaris painlio 

 and G. stygia (' Monogr. Pal. Phyllop.,' Pal. Soc, 1888, pp. 35 and 39 ; but in 

 Geratiocaris the angles are set in a contrary direction to what holds good in 

 Dithyrocaris (PL XX, figs. 2 b and 3^). 



From black non-calcareous shale in the Glebe Cement-stone Quarry, Kirkton- 

 holm Cement-works, East Kilbride. Mr. A. Paton's Collection. 



^ As in Hall aud Clarke's Mesotliyra, &e. 



