ANATOMY OF ARENICOLA ASSIMILIS. 773 



tins;uisliable from A. marina only bv this character. No 

 meution is made of other features which would have been 

 much more valuable as diagnostic characters^ but the 

 difference in the number of gills is accepted as a sufficient 

 ground for separating the specimens from xV. marina^ not- 

 withstanding the well-known liability to reduction (from 

 thirteen to twelve pairs) in the number of branchia) in this 

 species. As will be seen from the discussion below, it is very 

 probable that Ehlers' specimen does not belong to either of 

 these species, and that this is an example of the confusion 

 due to placing an implicit reliance on the value of external 

 features in discriminating species of Areuicola. On such 

 a variable and insufhcieut character as the number of gills 

 Ehlers bases his diagnosis of the Califoruian specimen, and 

 refers it to the species A. assimilis. This is the only- 

 evidence in support of his record of this species from Cali- 

 fornia, 



I have recently re-examined specimens of Arenicola 

 from a collection made by Professor Agassiz, near Crescent 

 City, California, sent to Dr. Gamble and myself from the 

 Harvard Museum, and identified by us (1900, p. 423) as 

 A. claparedii. These specimens are the more interesting 

 because they are accompanied by a label ^ indicating that they 

 have passed through the hands of Professor Ehlers, and that he 

 considered them to belong to a new species nearly related to 

 A. marina (= piscatorum). It is almost certain that these 

 are the same specimens which Ehlers has recorded as A. assi- 

 milis. There are five specimens, in three of which there 

 are twelve pairs of gills, the first situated on the eighth 

 chaitigerous segment. In each of the other two specimens 

 there are twelve gills on the left side (the first being on the 

 eighth segment), accompanied in one case by thirteen gills 

 on the right, the first being very small aud borne on the 

 seventh segment, while on the right side of the other 



' The wriling upon the label, whicli is now faint, is as follows: — 

 "Areuicola, n. sp. nahe |)iscal,or. 7 vor Segni. 12(13) Kieinenlrag. 

 Cahluiuieu (E. Ehlers)." 



