DITHYROCARIS TRICORNIS. 171 



PI. XXIV, fig. 1. Mas. Geo]. Surv. Scotl., b 3095 a. No. 12. 



Size. — Length of each moiety or valve 63 mm., including their posterior 

 spines. Breadth of the two valves 63 mm. 



Characters. — A nearlj^ perfect subquadrate carapace, consisting of two 

 moieties or valves. Ventral border elliptically rounded ; its thickened (double) 

 margin, as shown by its strong impression, was marked on the inside with longi- 

 tudinal delicate stride. On the edge it bore a fringe of strong oblique strige, or 

 compressed prickles, pointing backwards, especially at the posterior curve, where 

 they form a sharp serrated edge ; and they are less strong at the anterior region. 



The anterior margin had a gently curved medial hollow, where the two valves 

 meet. Dorsal junction along a ridged line, which is obscurely indicated as having 

 been a long, thin, separate (probably overriding) part of the test, with a narrow 

 flat flange along each side, somewhat like the ridge-tiles on the roof of a house. 



In PL XXIV, fig. 1, the flange on one side (spectators left hand) of the 

 dorsal ridge is definitely indicated by a thin line ; but on the other side it is 

 covered up by the black shale having been squeezed up over it inside the long 

 thin rugose ridge (juxtadorsal) parallel with the thick dorsal ridge. 



Besides the central ridge, there are two slight tuberculate ridges (juxtadorsal), 

 parallel and near to it, one on each side ; on the left reaching up to the nuchal 

 ridge, but interrupted on the right side. These two nuchal ridges are thin and 

 tuberculate, parallel, and close to the anterior part of the central (dorsal) ridge, 

 and between it and the sinuous cephalic (gastric or optic) ridge on each moiety of 

 the carapace. 



On each valve there is also a strong, straight, and rugose or tuberculate (meso- 

 lateral) ridge, between the dorsal ridge and the ventral border, passing from near 

 the gastric ridge to the posterior border just above the large spine at the postero- 

 ventral angle. There may have been also a middle posterior spine, terminating 

 the dorsal ridge, as in figs. 4 and 6, and PI. XXIII, fig. 1. The rugosity of all 

 the ridges is due to the sharp prominences of imbricated chevron-shaped scales, or 

 successive angular outgrowths of the test. 



The whole surface is sprinkled over with minute triangular tubercles, having 

 the apex jjointing backwards. The posterior coi-ners of the test, and some parts 

 of the ventral region, bear numerous round tubercles. 



All the ridges consist of numerous overlapping, raised, chevron-like layers of 

 test, pointing backwards. The surface of the specimen, PI. XXIV, fig. 1, is 

 partially obscured by thicker or thinner layers of black shale. On the outer 

 division of the moiety on the left hand there is mostly a thick layer, leaving bare 

 the postero -ventral angle and spine. On the inner division is a rather thinner 

 layer, leaving its anterior third bare, with its delicate reticulation. The inner 

 division of the right-hand valve carries a thick layer along the narrow area between 



