56 



MR. L. A. BORRADAILE ON THE 



much like those of Nebalia (text-figs. 38-43). Clearly, we have 

 here the mandible of Chirocephalas in process of further evolution. 

 The homology of the incisor process is more puzzling. No trace 

 of it can be found in the Entomostraca. In Lepidurus (text- 

 fig. 33) the end of the mandible, instead of being oval as in 

 Chirocephalus, is elongate, tapering to one end, and slightly 

 curved. It is crossed by deep ridges, few in number, and marked 

 each by a few coarse tubercles. The narrow end stands away 

 from the rest, bears three or four teeth instead of the ridges on 

 the wider part, and somewhat suggests an incipient incisor 

 process, but this is at the hinder end of the organ, whereas the 

 incisor process of Malacostraca is anterior. It seems likely that 



Text-figure 31. 



End view of mandible of Nebalia sp. 

 For lettering see p. 71. 



the biting-surface of the mandible of Notostraca represents one 

 of the two sets of ridges found in the same position in Chiro- 

 cephalus. In G alarms (text-fig. 35) the condition is not dissimilar. 

 The biting surface is narrow, elongate, tapering towards the 

 ends, especially towards the hinder end, and crossed by coarse 

 ridges, variously tuberculate. At the anterior end stands a 

 structure Avhich at first sight a little recalls the incisor process, 

 but in an end view of the mandible this is seen to be only the 

 first ridge, removed a little from the rest and connected with 

 them by a flange. The mandible of Cypris (text-fig. 34) is of 

 the same type, but the anterior ridge is less outstanding and not 



