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THE AMERICAN NATURALIST [Vol. LII 



that their notes were delivered independently of the rhythm of 

 this particular group. From observations of the stridulations 

 of other groups in this same colony, I am of the opinion that it 

 is not unusual for these locusts to develop a rhythmic synchro- 

 nism in small groups. 



It would be interesting to know why some species of locusts 

 and crickets possessing the intermittent habit of stridulation 

 tend to develop a more or less perfect rhythmic synchronism 

 while others do not. Although this is true of the two crickets, 

 (Era a thus niveus and Cyrtoxiphacolumbiana,! have been unable 

 to note any synchronism in the chirpings of the common arboreal 

 cricket, Orocharis saltator. Although large colonies of these 

 crickets may often be heard in stridulation, each individual 

 appears to stridulate in its own leisurely manner independently 

 of its fellows. 



H. A. Allard 



ON THE PIGMENTATION OF A CLYPEASTROID, 

 MELLITA SESQUIPERFORATUS LESKE 1 



The common clypeastroids, Echinarachnius and Mellita, when 

 adult, are characteristically of a brown or (in the former spe- 

 cies) reddish-brown color. This seems to be general throughout 

 the group. Taxonomic lists contain, however, numerous refer- 

 ences to a greenish coloration of the test of these animals. 

 When preserved in alcohol, or when dried, either after fixation 

 in alcohol or after killing with fresh water, these sand dollars 

 usually assume, for a time at least, a somewhat greenish color. 

 Clark (1899, p. 118) says that specimens of Mellita sesquiper- 

 foratus Leske {=sexforis A. Ag.) collected at Jamaica were 

 delicate olive green [when alive, I infer, though with doubt]. 

 He also gives the coloration of specimens of this species obtained 

 in Porto Rican waters as "usually light olive green (rarely 

 brown) when alive." At Bermuda living individuals of this 

 species are, he says, invariably brown, with no hint of green 

 about them, either on the external surface or in the viscera. 

 When killed in alcohol, however, they become green, and green- 

 ish pigment is dissolved by the fluid. This is also true of Echi- 

 narachnius parma (Clark, 1904, p. 564; Coe, 1912, p. 111). 



Now, examination shows that there is at the bottom of this 



i Contributions from the Bermuda Biological Station for Besearch, No. 90. 



