200 BRITISH PALAEOZOIC PHYLLOCARIDA. 



organic origin on eacb face, but not exactly corresponding one with tbe other in 

 position, size, or shape. One side shows a distinct form (Figs. 12 a, 12 b), like 

 that of the dorsal edge of a compressed bivalved form (50 mm. by 23 mm.) ; the 

 other side shows a flatter and less shapely organism (Fig. 13), like a portion of 

 the crushed valve of a Dithyrocarid. 



Fig, 11. — The concave specimen on the slaty schist from Coesmes shows either 

 an impression of the outside of two expanded valves, or at least partially (as 

 indicated to some extent here and there by traces of a black shining film) the 

 inside of the two valves. The cast of matrix once filling the cavity, and bearing 

 probably some other details of the animal's structure, has been lost. 



This has some analogy to specimens of flattened valves of Bithyrocaris illus- 

 trated in Pis. XVIII and XXVIII. 



In the two flattened valves (Fig. 11) the dorsal edges are in contact ; not quite 

 clearly defined, however, one from the other, on account of a partial overlap, or 

 some other discordance, along the middle line. Thej'^ open out one from the other 

 at the ends, but more definitely at one end than the other. Both of the extre- 

 mities have been damaged. 



Being greatly though unequally flattened by pressure, they present an aspect 

 comparable, as far as general outhne is concerned, with fig. 5 a, PL XXVIII, 

 which is an impression of the outer surface of the expanded valves oi Ditlnjrocaris 

 testiidinea ; also to some extent with figs. 3 a and 6 of PI. XVIII, illustrating the 

 outside aspect of the expanded valves of D. oralis and D. graiiidata. The fuller 

 lateral curve, however, is in the anterior rather than in the posterior portion. 



The upper end in Fig. 11 has a serrated edge, which has only a feeble homologue 

 in figs. 1, 4, and 6 of PL XVIII, and in fig. 3 of PL XXII. 



At the lower end of Fig. 11 each valve narrows much more quickly than in any 

 of our other figured species of Dithyrocaris, and does not bear clear evidence of 

 the usual ventral terminal spines ; if ever j^resent, they have been lost, or crushed 

 out of recognition. 



The longitudinal foldings or wrinklings of the test in each valve are probably 

 homologous with those in Gah/ptocaris striata, PI. XVII, fig. 7, and C. Richteriana, 

 PL XXII, fig. 2. The ends of the valves in these two figured species are too im- 

 perfect for comparison with those of Lebescontia. 



Figs. 12a and 12 b. — On the hand-specimen from Renaze, the more distinct fossil 

 of the two, lying back to back, suggests at first sight a comjDarison with the doi'sal 

 aspect of a closed bivalved test of some Dithyrocarid, compressed, but retaining 

 some lateral convexity (Fig. 12 a). It has a terminal appendage, or blunt spike, 

 overlying a displaced portion of the hinder end of the right valve. The posterior 

 process diffei'entiates this form from Fig. 11, unless it has been lost in the latter. 



The upper end of Fig. 12 a retains no evidence of the denticulate margin 



