DITHYROCARIS. 131 



higher angle in some that may be bivalved. The hinge-line is simple, and the 

 valves easily separated. 



Besides a dorsal (middle) ridge, appertaining to both valves, each valve has a 

 median ridge (the mesolateral) . Generally a pair of short sigraoidal rugose ridges 

 occur on the cephalic or gastric region, and another small pair further back and 

 nearer to the dorsal edge (the nuchal ridges). Another ridge is sometimes present, 

 near to and parallel with the dorsal on each valve {the juxta-dorsal ridges). 



In shape the tests vary from orbicular to suboblong, and the valves from 

 suboblong to semicircular. Each valve has usually a slight projection on the 

 curvature of its anterior edge. A neat fringe, or occasionally a cord-like border, 

 of oblique strige, pointing backwards, ornaments at least a part of the convex 

 outer (ventral) border; and this edge, curving backwards, is prolonged over and 

 beyond the posterior border in a strong sharp spine or spike. The extreme 

 verge of this border was turned downwards or inwards. 



In some tests there is good evidence that the dorsal or ceutral junction-line 

 is overlapped by a rugose ridge of minute angular imbricating flakes of shell- 

 matter, forming plicge or wrinkles (just as in the other ridges), and ending in a 

 strong triangular point. In one or more folded specimens and in some separate 

 valves this middle ridge, bearing an obliquely spinous fringe, lies on its side, and 

 thus shows its crest. This dorsal ridge is seen in some specimens to lie evenly 

 with the moieties, but in others clearly to override the two dorsal edges placed in 

 apposition beneath it, and not to lie between them as in the figures and description 

 of Mesothi/ra by Hall and Clarke, ' Nat. Hist. New York Palaeout.,' vol. vii, 1888. 



The surface of the two valves or moieties may be smooth, punctate, or granu- 

 lated ; and some are traversed by oblique wavy lines (analogous to lines of growth). 

 Interstitial sculpture is also present. Some allied forms bear thin longi- 

 tudinal ridges. In many instances abdominal segments and a trifid caudal 

 appendage are present. Gastric teeth frequently occur, sometimes in their place 

 in the cephalic region, but more often free and scattered in the shales. 



It has been observed in the ' Geological Magazine,' vol. viii (1871), p. 106, 

 that " it is exceedingly difficult to decide whether this form [Choenocaris tenui- 

 striata^ had its valves widely expanded, as in the recent Apti,s, and probably 

 in the fossil Dithyrocaris Sconleri ; or whether, as in Nebalia and Ceratiocaris, 

 they were folded down upon the sides of the animal's body. The distinction 

 seems to be an important one ; but the frequent occurrence of the united 

 expanded valves of Ceratiocaris in the Upper-Silurian shales of Lesmahagow 

 often reudei's it difficult to decide as to the actual and normal degree of expansion 

 or of folding down of the lateral borders of these crustacean shields during the 

 lifetime of the animal." 



The partially open or not quite closed carapace of Choenocaris Youngii, 



