322 



CRUSTACEA. LEPTOSTRACA 



Caryocaris (fig. 140). Carapace bivalved, pod-like, narrow 

 smooth, rounded at one end (pro- 

 bably the posterior), truncated at 

 the other. Arenig Rocks. Ex. C. 

 wrighti. 



Dithyrocaris. Carapace large, 

 bivalved, with a narrow anterior 

 notch ; rostrum unknown. Each 

 valve semi-oval, truncated behind, 

 with a median longitudinal ridge ; 

 another ridge at the dorsal margin 

 where the valves join. Surface 

 often with pits or granules. Ex- 



Fig. 140. Caryocaris wrighti, 

 Arenig Rocks. Natural size. 

 The abdomen has not been 

 found attached to the cara- 

 pace as shown above ; some 

 authors consider that the 

 broad end of the carapace is 

 anterior. 



posed part of abdomen short, with 



a narrow, sharply-pointed telson bearing on each side a spine-like 

 appendage. Devonian and Carboniferous. Ex. D. colei, Carboni- 

 ferous. 



Discinocaris. Carapace sub-circular, slightly convex, formed 

 of one piece with a notch in front in which the triangular rostrum 

 is placed. Surface with concentric linear ridges. Silurian. Ex. 

 D. browniana, Llandovery. 



Aptychopsis. Similar to the last, but carapace divided into 

 two parts by a median suture which starts from the rostral notch. 

 Silurian. Ex. A. lap worthy Llandovery. 



Distribution of the Leptostraca 



The Leptostraca are all marine, and live mainly in 

 shallow water or at moderate depths. In Britain the 

 earliest representative is Hymenocaris, found in the 

 Lingula Flags ; Ceratiocaris occurs in the Tremadoc Beds 

 and ranges on to the Carboniferous, but is most abund- 

 ant in the Silurian. Caryocaris is characteristic of the 

 Arenig Rocks. Aptychopsis and Discinocaris occur in the 



