166 BRITISH PAL/EOZOIC PHYLLOCAKIDA. 



Size. — Moiety or valve 68 mm. long; including posterior spine 74 mm. 

 About 35 mm. wide. The whole carapace was originally about 70 mm. wide. 



Characters. — The flattened carapace (PI. XXIV, fig. 2) is suboval, anteriorly 

 contracted, and showing a relatively broad, shallow, central notch, where the 

 edges of the two valves (or lateral moieties) turn slightly inwards and backwards 

 to their junction. 



In the separate subelliptical moieties the ventral edge is elliptically curved 

 and depressed. The dorsal edge is straight, and provided with a rugose ridge 

 (PI. XXIV, fig. 2), which is seen in its side view (PI. XXIII, fig. 1) to be crested, 

 and to erid behind in a broad triangular spine. The dorsal edge of each valve, 

 when in apposition seems to have been overridden by the straight, narrow, 

 rugose crest, which had narrow lateral flanges, and constituted the medial or dorsal 

 ridge of the carapace. There is also on each valve a straight tnesial rugose ridge, 

 besides a shorter, coarser, and rather sinuous cephalic ridge, a weak juxtadorsal, 

 and a thin, short, nuchal ridge. This last is more distinct in PI. XXIV, fig. 4, 

 than in fig. 2, where the two small nuchal ridges, usually parallel with each other 

 and with the dorsal ridge betweeu them, are somewhat displaced, probaiily by 

 unequal pressure at that spot, as evidenced by the apparent local disturbance of 

 the specimen. 



The mesolateral ridges are attenuated in front, aud curve towards the cephalic 

 ridges in PL XXIV, fig. 2 ; but in fig. 4, and PI. XXIII, fig. 1, they are straight 

 as far as they go. They end posteriorly just above (in front of) the postero- 

 ventral notch and spine, PI. XXIII, fig. 1, and PI. XXIV, fig. 4. This spine is 

 relatively long, triangular, flat (?), and sharp ; and is continuous with the 

 depressed edge of the ventral margin. This, marked off' by a smooth, thin ridge 

 (the I'eal solid rim of the valve, partially preserved in figs. 1 and 4), becomes 

 narrower forward, and is furnished with a fringe, or strongly and obliquely 

 striated border along the hinder two-thirds of its length, perfectly shown in 

 Portlock's fig. 2, pi. xii. This narrow, flat, or depressed portion of the ventral 

 margin appears to have been a free edge, and to have been longitudinally striated 

 on its under side. 



The fringed border of the left moiety is for the most part preserved as a 

 narrow whitish rim of the test, with the stri* lying close together, adpressed and 

 almost cord-like, with a partial film of shining black shale, which emphasises the 

 minute granulation on each fibre of the fringe. 



The posterior edge of the valve is nearly straight between the dorsal and the 

 (larger) ventral spine (PI. XXIII, fig. 1, and PI. XXIV, fig. 4). 



In the specimen illustrated by fig. 2, PI. XXIV, the surface has a faint and 

 delicate reticulation in the anterior part of each valve between the cephalic and 

 dorsal ridges. It is much obscured elsewhere in the compressed shale, which has 



