38 



MB. C. BAILEY ON THE STRUCTURE ETC. 



In Naias graminea the type of spine is similar^ but it 

 differs from that of N, flexilis in having a bi-celled base 

 whose sides unequally support the spine. The lowermost 

 of the two basal cells diverges^ at its upper end_, from the 

 line of the leaf-margin^ so as to wholly support the lower 

 end of the spine (see fig. 47). The uppermost cell^ on the 

 other hand_, acts as a support to the inner side of the spine 

 for fully one half its length ; it also partially underlies the 

 upper end of the lowermost basal cell_, and thus its three- 

 sided profile fills up the axil of the spine and adds con- 



Fig. 45- 



Fig. 46. 



siderably to its rigidity^ as compared with the arrangement 

 in A^. flexilis (comp. fig. 45) . Occasionally a third cell 

 makes its appearance^ as shown in fig. 48^ and not infre- 

 quently there is an auxiliary spine between the upper 

 supporting cell and the original spine (see fig. 49) . In 

 all these cases^ however^ the axillary^ or uppermost^ basal 

 cell distinguishes the type of tooth from the characteristic 

 tooth of N. flexilis. Cesati gives figures of the dentition 

 of these two species in plate ii. of ' Linnsea/ vol. xxxvi. ; 



