6 University oj Michigan 



baking the shells to make the quick-lime used to soften maize 

 in the preparation of tortillas, the national Mexican bread- 

 stuff. The shells from the Laguna de Catemaco (H, vii, d) 

 were obtained, through the intervention of the Spanish gen- 

 tleman whose hospitality we enjoyed, from a pile (all of one 

 species) at the edge of the town of Catemaco. They probably 

 came from the mucky-bottomed portion of the shore, near the 

 outlet. All of the shells from the Rio San Juan, except the 

 one specimen of Anodonta, were bought (for one peso) from 

 a peon at the village of Cuatotolapam, on the hacienda of the 

 same name. He informed me, through the medium of Mr. 

 Thomas La Rue, the subgerente of the plantation, that they 

 were obtained, during low water, from a depth of a few feet 

 in the Rio San Juan, near the town (H, vii, c). 



UNIONIDAK 



Aniblciua (Megalonaias) nickliniana ( Lea) (1834). — One 

 small left valve, from the Rio San Juan (H, vii, c). Measure- 

 ments^ : length, 86 mm. ; height-index at beaks, 75 per cent 

 (64.5 mm.) ; height-index at wing, 80 per cent (69 mm.) 

 diameter-index, 37 per cent (32 mm.). 



From the specimens labeled Q. eightsii (Lea) in the A. N. 

 S. P., that form, which Simpson (1914) regarded as a syno- 

 nym of Q. heros (Say), resembles some specimens of the latter 

 species less than it does Q. nickliniana. At least, it looks as 

 if the range of variation in Q. heros, as used by Simpson, is 



1 The measurements of the naiades in this paper are given in the 

 same order and with the same significance as in Simpson (1914), except 

 that, instead of the height and diameter, the height-index and diameter- 

 index are given. The index of the height is taken as the height divided 

 by the length ; that of the diameter, the diameter divided by the length. 

 Both indices are expressed as percentages. Except in the case of quo- 

 tations, the indices are followed (in parentheses) by the actual dimen- 

 sions in millimeters. 



