86 BRITISH PALEOZOIC PHYLLOCARIDA. 



upper part of the posterior region there are some small suboblong pits, which may 

 possibly indicate the bases of former prickles or tubercles. 



2. Saccocaeis minor, T. B. J. and H. W., 1891. Plate XIV, figs. 7—9, and fig. 10 (?). 



Saccocaeis minoe, T. B. Jones and H. Woodward. Eeport Brit. Assoc, for 1890 



(1891), p. 424, figs. 1—17. 

 — — Woods. Catal. Type Foss. Cambridge, 1891, p. 137. 



On a large piece of the " Upper Shale ( = Daear-fawr Shale) west of the Craig 

 known as Craig-yr-hyrddod, Arenig," North Wales, kindly submitted by Professor 

 T. McKenny Hughes, F.R.S., for examination, are numerous, and at first sight 

 somewhat obscure impressions of a bivalved Phyllocarid, together with some 

 body-segments of the same. The rock is "the top bed of shale tangled among 

 the porphyries of the Mountain Arenig. It is, therefore, the highest fossiliferous 

 zone of the Arenig of Arenig." The slab, measuring 18 by 10 inches, and half 

 an inch thick, consists of a hard, dark-coloured, fine-grained flagstone (dark-blue 

 within and weathering dull rusty grey), not argillaceous nor calcareous, made up 

 of minute, fragmentary, crystalline particles. One edge is straight and ragged, 

 and the opposite edge is rounded, as if it had been a part of a large fissile 

 concretion. The slab separates horizontally into two parts, and the counterpart 

 surfaces are covered with the fossil impressions, which are mainly convex on one 

 of the faces, and concave on the other. One larger convex cast (PI. XIV, fig. 7) 

 lies almost alone on the rusty weathered back of the piece that bears the concave 

 impressions. These carapaces and abdominal segments are merely dark films, 

 more or less flattened, and squeezed across their length. Some, however, among 

 the numerous individuals are less distorted by pressure, especially the one (fig. 7), 

 which is isolated on a different (outer and broadly rippled) surface of the stone. 



The crowded fossils lie mostly oblique to the long axis of the stone, near to 

 each other, often close together, more or less parallel, and generally with the same 

 end in one direction. On the plate at page 425 of the Report for 1890 some of 

 the best preserved specimens were selected and outlined just as the individuals 

 lie on the stone; sometimes, as figs. 1 and 2, 9, 10, and 11, 7, 15, 16, and 17, in 

 groups. See also PI. XIV, fig. 9. 



These carapace-valves are more or less oval-oblong in outline, but often 

 imperfect, and in nearly all cases modified in shape by pressure. 



The largest individual (PI. XIV, fig. 8), 37 mm. long and 22 mm. high 

 (broad), having nearly its original shape, has its upper and lower edges slightly 

 convex and nearly parallel ; the upper (dorsal) edge is somewhat more fully 



