TRIBE DIASPIDINI 303 



ally represented by both mesal and lateral groups, sometimes by 

 only a single group, mesal or lateral. There is considerable varia- 

 tion in the number of oraceratubae in the various groups in differ- 

 ent species and some use has been made of this difference in number 

 in differentiating species. 



The lamaceratubae are never very prominent in the pygidia 

 of this tribe. They are slender and extend through each plate, 

 their oraceratubae located at the distal end, as in the plates found 

 in the pygidia of other species. 



The vulva is a transverse slit, which is usually located near 

 a transverse line drawn through the pregenacerores and 'post- 

 genacerores, that is, it is approximately central in position. There 

 is a slight variation from this position, in some it is a short dis- 

 tance cephalad and in others a short distance caudad of this line. 



While the vulva is fairly constant in position, the anus seems 

 always to be extremely variable. It may be located caudad of a 

 line drawn through the caudal margin of the postgenacerores or 

 in the cephalic portion of the pygidium, usually cephalad of a 

 line drawn through the cephalic ends of the pregenacerores or 

 it may be located anywhere between these lines. In a number 

 of species it is superimposed over the vulva. 



The vulva is typically surrounded by five groups of gena- 

 cerores. In a few genera the number may be increased to seven 

 or more groups. The number of cerores in each group is vari- 

 able. There is usually about eight mesogenacerores, the number 

 varying from two to thirty or more. The groups of pregena- 

 cerores may contain a few cerores, twelve or a large number, 

 sixty. There is also considerable variation among the postgena- 

 cerores, the number varying from one to forty. The mesogena- 

 cerores are sometimes wanting and in several genera all the gena- 

 cerores are wanting. Green records the presence of two more 

 or less definite glandular scars. These are reticulate and located 

 cephalo-laterad of each group of pregenacerores. They are con- 

 sidered as homologous with the groups of genacerores, in fact he 

 describes the occurrence of cerores in one of these glandular scars. 



The marginal setae of the pygidium are frequently compara- 

 tively short and stout, so that they extend as rigid projections 

 and, for this reason, are easily mistaken for plates and have 

 been described as such. This is particularly true of the two 

 prominent setae located in the median incisura. These setae 

 instead of being located on the dorsal side of the median pair 



