314 CRUSTACEA. OSTRACODA 



are seven pairs of appendages, which can be protruded 



when the shell is opened. In some of the marine forms 



the shell is notched anteriorly so as to allow the antennae 



to pass through when the shell is closed. The head 



carries two pairs of large antennae which are used for 



locomotion, one pair of mandibles, and two of maxillae. 



The thorax has two or three pairs of appendages, which 



are not leaf-like. The abdomen is rudimentary and is 



without appendages ; it terminates either in a fork or in a 



spiny plate. Respiration takes place by means of the 



general surface of the body. The carapace is in almost 



all cases the only part which occurs fossil; its surface may 



be smooth or variously ornamented. 



Leperditia. Carapace thick, smooth, convex, elongated, a 

 little higher posteriorly. The right valve larger than the left. 

 Hinge-line straight. There is a small tubercle (eye-spot) placed 

 anteriorly near the hinge ; and posterior to it is a slightly elevated 

 circular area. Cambrian to Carboniferous. Ex. L. kisinge?'i, Silu- 

 rian ; L. okeni, Carboniferous. 



Primitia. Carapace generally equivalve, convex, oblong or 

 ovate. Hinge-line straight. Each valve has a transverse groove 

 which starts from the hinge-line. Cambrian to Carboniferous. Ex. 

 P. strangulata, Bala Beds. 



B eyrie hi a (fig. 136). Carapace elongated, inflated, posterior 

 border a little higher than the anterior ; 

 dorsal border straight, ventral border semi- 



circular. Two or three large furrows 



pass from the dorsal towards the ventral 



edge ; the parts between the furrows 



are convex and often tuberculate, the 



middle part being the smallest. Cam- Fi °' ^ Beyrichiacom- 



. ~ .„ -r-i r. 7- phcata, Bala Beds. 



bnan to Carboniferous. Ex. B. complicata, ^he lower figure 



Llandeilo and Bala. shows the dorsal 



__ _. .I, aspect of the united 



Entomis. Carapace equivalve, al- valves, x 2. 



mond-shaped, with a deep transverse 



furrow which passes from the dorsal border (a little in front of the 



