THE TARANTULAS 131 



its burrow with a trap door. The remaining genus, Antrodiaetus, 

 has numerous species; they are widely distributed in the southern 

 states right across the country, and in the mountain states and the 

 Pacific Northwest are the commonest mygalomorph spiders. 



An important feature of this group is the possession of a distinct 

 rake on the chelicerae. For this reason they have long been placed 

 among the true trap-door spiders of the family Ctenizidae, a group 

 they resemble closely, but one that has taken an entirely different 

 route in its development. In Antrodiaetus the anterior lateral spin- 

 nerets have been lost, but in the other two genera the six spinnerets 

 are all present, with the same arrangement as in At y pus. The pres- 

 ence of two, three, or four well-marked tergites at the base of the 

 abdomen in both sexes is invariable; these are strikingly large and 

 distinct, set with rows of transverse setae as in the liphistiids. 



The turret spider, Atypoides riversi, lives in the foothills of the 

 Coast Ranges of California, and is found in abundance along shaded 

 streams and in thickets in the San Francisco Bay region. Its turrets 

 are well-known objects. They are ordinarily open at the top, lack- 

 ing completely a closing flap or trap door, but on occasion will be 

 completely spun over and closed with silk and debris. The burrow 

 is very long, usually inclined, and is lined completely and rather 

 heavily with white silk. The aerial portion may be only a short 

 chimney, but quite often there is a long tube, which, penetrating 

 thick grass, moss, or debris, finally terminates in the expanded white 

 lip of the turret. The spider takes whatever building materials are 

 handy leaves, small twigs, moss, bits of lichen, pine needles and 

 fastens them on the outside of the silken collar. Often most in- 

 geniously constructed, the turret provides an excellent lookout for 

 the spider, which sits in the entrance at dusk and catches the insects 

 that come within its reach. 



The turret spiders are about half an inch long, with yellowish 

 brown carapaces and darker brown or purplish abdomens. A re- 

 markable feature of the male is the presence of a long, projecting 

 process on each chelicera, which probably is concerned with mating 

 since no similar spur exists in the female. The tiny anterior lateral 

 spinnerets are composed of a single joint, and, judging from their 

 reduced size and lack of spinning equipment, are rapidly being 

 aborted. The median groove of the carapace is a linear impression. 

 The dorsal tergites on the abdomen are well marked, three being 

 represented clearly in each sex. 



The second exclusively Calif ornian genus is Aliatypus, which 



