62 Annals Entomological Society of America [Vol.11, 



cylindrical to a flat boat-shaped form, hollowed out on the side 

 toward the palpus, would very evidently be of service in shield- 

 ing the bulb from mechanical or other injuries. 



(b) Change in the position of the bulb, from an exposed posi- 

 tion at or near the tip of the cymbium, is correlated with the 

 change in the form of the cymbium mentioned under (a), and 

 enables the bulb to make full use of this alteration. 



(c) The distensibility of the articular membrane makes a long 

 or projecting bulb less necessary, since it may be projected at the 

 time of copulation, and only then. 



(d) The purpose of splitting the bulb into segments or plates 

 is, in the majority of cases, less evident, but in one large family, 

 the Lycosidae, it enables the bulb to be closely folded together 

 in the alveolus. 



The whole evolution of the male palpus, therefore, appears 

 to hinge on the one adaptation, and the different modifications 

 in the form of the palpal organ are accordingly only illustrations 

 of the different means which may be employed to serve this end. 

 i\bove all stands out the constant mechanical adaptation or fit- 

 ting together of bulb and cymbium to one another, often carried 

 to an extraordinary degree. When the palpus is at rest, the bulb 

 and cymbium together, in the majority of the Dipneumones, form 

 a compact, more or less ovoid mass. This is brought about, gen- 

 generally speaking, in one of five ways, thus dividing the American 

 spider families of the sub-order Dipneumones into at least five 

 divisions. The lines of separation between these divisions, are, 

 of course, not always sharply marked, since transitional forms 

 occur, but the types themselves are nevertheless quite distin- 

 guishable. They are as follows: 



1 . First of all are those families in which the structure of the 

 male palpus dift'ers little, if any, from that described for the 

 Tetrapneumones. These families are: 



Filistatidae. 

 Dysderidae. 

 Sicariidae (Scytodes). 



The Pholcidae are probably also to be placed here. It is to 

 be noted, that the cymbium of Dysdera crocata shows e^'idence 

 of the flattening so constantly present in the higher families. 



2. Fig. 2. The principal axis is greatly shortened, the bulb 

 is in cross section nearl}^ circular, and sinks into the circular 



