EXTOMO LOG ICA L Co N TRTB UTIONS, 



123 



black bands laterally and ventrally, and by the yellowish shade rest- 

 ing on the incisures laterally. 



It is possible that tlie minute description which Mr. Riley proposes 

 shortly to give oi^ grata, drawn from a large number of living larva3, 

 when conjpared with the detailed description which I have endeavored 

 to give of i/?iio, may develop such points of difference as will permit 

 of tlie ready recognition of these two forms, peculiarly interesting 

 from the close resemblance existing between them. 



Since the above has been put in type, I have been able to compare 

 my two examples of ^. unio larvae with six alcoholic examples of 

 E. grata, and, as the result of such comparison, I am compelled to 

 disagree with Mr. liiley as above quoted, for while the two are very 

 similar in their ornamentation, yet I find such differences that (pro- 

 vided the features to be referred to prove constant in larger numbers) 

 I would have no difficulty in selecting a single mature individual of 

 either species from among a thousand of the other. Through the 

 kindness of Mr. Riley, I am able to accompany this paper with an 

 excellent figure from drawings made by himself of the larva of 

 E. grata, and also (for the first time) a representation of the beautiful 

 e^^ of this species. At a the larva fig. 5. 



is shown of its natural size ; at h, 

 one of the segments (the fifth) en- 

 larged ; at t', the ordinary ornamen- 

 tation of the collar, differing in 

 some examples by the addition of 

 several (to the number of eight) 

 central dots ; at d, the usual marking of the hump on the eleventh 

 segment ; e represents the Qy:^g as seen from above, and /* is a side- 

 view of the same (natural size shown with the enlarged figures). 



The following are the principal differences that I find in the two 

 species : 



Contracted by their preservation in alcohol, the two itnio larvae 

 average in length 1.05 in. ; the six grata 1.29 in. They differ in 

 form, in that the latter presents much the more prominent hum]) on 

 the penultimate segment, and is angulated at that point to a degree 

 that were it a vertebrate, it would suggest the idea of its terminal 

 portion dragging from having been broken at that point ; in Knio 

 the humjj is moderate and the peculiar angulated form, well repre- 

 sented in the figure, is not seen. 



Unio is the more heavily marked with black, both in its bands 



