124 TWENTY-SIXTH REPORT ON THE STATE MUSEUM. 



and spots. In none of the examples of grata are the black bands 

 broader than one-half the width of the intervening white ones, while 

 in one unio their average width is nearly double that of the white. 

 The spots on the head are the same in position in both species, but 

 are smaller in grata. In that species there are usually two distinct 

 piliferous spots on the base of the clypeus ; in two of my examples 

 these are confluent, running together by slender projections in a 

 broad V-form : in unio the two are united as a band across the clypeus. 

 In unio a black spot, broadly rounded beneath, following the curved 

 line of the ocelli, and tapering to a point above, incloses the four 

 superior ocelli : this is not present in grata, but in two examples some 

 of the ocelli are indistinctly annulated with black.' 



At c, in Fig. 5, the spots on the collar of grata are faithfully repre- 

 sented in position, but their size might have been slightly enlarged. 

 In unio, the four spots of the anterior row are separate, but those of 

 the posterior row, from their greater size as compared with grata, 

 are confluent, except the two medial ones : in grata, these spots are 

 separated by spaces varying from one diameter of a spot to two and 

 one-half diameters. 



The spots on the caudal hump of grata, shown at d, in the figure, 

 are isolated, while in unio those in each row are connected by the 

 black band to which they are united. 



Similar comparisons might be instituted of all the other spots of 

 the two species, but the above may indicate their differences. The 

 feature which should serve better than any other to distinguish unio 

 is the blackish coloring (its outline not permitting its designation as 

 a stripe), above the prolegs and continued on the two following seg- 

 ments, the three piliferous spots above the prolegs being connected 

 with it ; this is entirely wanting in grata. It results, apparently, 

 from the increasing breadth and coalescence of the black bands as 

 they descend to the ventral region. In one of the examples, the ven- 

 tral region of the proleg segments is essentially blackish, which fea- 

 ture was also observed in a number of the living larvae, according to 

 my recollection and that of Mr. Meske, who also collected the larvae 

 and bred from them several imagines. 



The differential features above indicated are not entitled to the 

 reliability that would attach to them, were they drawn from living 

 examples ; but should they prove to be sustained by future observa- 

 tions, there need be no necessity of failing in the determination of 

 these species, when either may chance to be collected. 



