THE NAUTILUS. 53 



sulcata Krauss. As Simpson believes also to be typical his specimen 

 with purplish nacre, further investigations will be necessary, how- 

 ever, as it is probable that the species shows some variability, and 

 that Tetraplodon schombergianus is a synonym. 



Conformable with the conceptions here exposed, we may consider 

 the super-family of Naiades as composed of two families : UnionidcB 

 and Diplodontidcu. The former, the only one occurring in North 

 America, may be divided into the following three sub-families : 



Uniosid.e with the genera: Anodonta, Lastena, Gonidea, Ano- 

 dontoides, Pegias, Arcidens, Symphynota, Alasmidonta. Hemilastena, 

 Margaritana, Unto. 



QuADUULrNyE with the genera : Quadrula, Pleurobema, Obovaria, 

 Cyprogeuia, Obliquaria, Dromus. 



Lampsilin.e with the genera : Lampsilis, Truncilla, Micromya, 

 Medionidus, Nephronaias, Glebida, Plagiola. 



As shown by the arguments given, the opinions of Mr. Simpson, 

 in many essential points, are in accordance with those of the writer, 

 and I thought it useful to publish such of my ideas as do not accord 

 with Mr. Simpson's, in order to submit them to examination in the 

 discussion which a book of such great importance will naturally 

 provoke. 



S. Paulo (Brazil) 10 May, 1901. 



AN ABNORMAL CHITON. 



In September of last year Mr. Hemphill sent me a photograph of 

 an abnormal specimen of the common Californian species Ischnochiton 

 (Maugerella) conspicuus Cpr., the divergence from the ordinary 

 form consisting in its having only six valves. The length of the 

 specimen (photo) is about 70 and the breadth 38 millimeters. There 

 is no indication of pathologic deformity; the proportionate size of 

 the valves compared with the size of the shell as a whole, exhibits 

 no eccentricity. Cooke, in the Mollusca volume of the Cambridge 

 Natural History Series, remarks that "seven-valved monstrosities 

 very rarely occur." Sykes has recently reported and figured a 

 three-valved individual. In the hundreds of Chitons of various 

 species that I have collected on the West Coast I have never de- 

 tected an example with less than the usual number, though it is pos- 

 sible in handling a large number, especially of the smaller forms, to 



