DESCRIPTION OF GENERA AND SPECIES. 257 



ABDOMEN: Subcylindrical, widest toward the base, rounded and somewhat narrow in 

 front, where it overhangs the cephalothorax ; somewhat narrowed at the apex, which slightly 

 overhangs the spinnerets; dorsum but little arched, yellow, with veined brandling lines 

 from the middle to apex and scalloped marginal lines; the surface thickly covered with 

 short silvery hair, which gives the organ a metallic white color; the skin color is yellow. 

 The epigynum (Plate XXII., Fig. la) shows simply a thickened flap, obtusely triangular, 

 without a scapus, and set opposite a conical process upon the posterior side of the genital 

 cleft, which is thus inclosed between two liplike elevations. 



DISTRIBUTION: Southern California. The habitat of this species is somewhat in doubt. 

 Dr. Marx received it from Professor Ulysses Brown, Edinburgh, Scotland, and it was con- 

 tained in a broken box, the label of which was mutilated ; but in the same box were 

 several bottles, labeled "Group of southern islands, Santa Barbara, California." 



GENUS THERIDIOSOMA, CAMBRIDGE, 1879. 



This genus forms a connecting link between the Orbweavers and Theridioid spiders. It 

 was created by Cambridge for the kindred, and perhaps identical, species Th. gemmosum, 

 and assigned to the Retitelaria;, but was transferred by the writer to the Orbitelarise, both 

 on account of its habits and structure. 1 The individuals are very small, of delicate colors, 

 and in general form resemble the species of Theridium. The cephalothorax is cordate, 

 truncate at the base, the head much elevated, the face wide and projecting in front, the 

 corselet grooves and cephalic suture are distinct, the skin glossy. The sternum is cordate 

 or obtusely triangular; almost as wide as long. The labium is short, scarcely one-third the 

 height of the maxilla}, which are as wide as, or wider, than long, in the female. The 

 eyes are divided into three groups, the central quad longer than wide, and the greatest 

 width behind ; the midrear eyes being much the larger, and placed close together, rela- 

 tively much closer than the midfront. The space between the sidefront and midfront eyes 

 is twice the intervening space of the latter, or about equal to the area thereof; the clypeus 

 is high ; the front row of eyes is aligned, or a very little recurved ; the rear row decidedly 

 procurved. The legs are 1, 2, 4, 3; rather stout; scantily pubescent; provided with sharp, 

 bristlelike spines. The mandibles are long and conical. The abdomen is a rounded oval, 

 the dorsum high, arched to the distal spinnerets, and is carried by the spider in a nearly 

 vertical position. The epigynum is a large vaulted atriolum, without a scapus, occupying 

 half the width of the venter. The male resembles in color and markings the female, 

 though somewhat smaller in size. The palpal bulb is globular, and the accessory organs 

 complex and much lengthened. This spider makes an orb web of the characteristic Epei'roid 

 type, but captures its prey by means of a trapline, which it uses somewhat in the fashion 

 of Hyptiotes, by alternate rapid tightening and releasing. 



No. 105. Theridiosoma radiosum McCooK. Plate XXVII, Figs. 8, 9. 



1881. Epeira radiosa, McCooK .... Proceed. Acad. Nat. Sci., Phila., p. 163. 



1884. Microepeira radiosa, EMERTON . . N. E. Ep., p. 320, pi. xxxiv., Fig. 7. 



1889. Theridiosoma gemmosum, McCooK, Amer. Spiders and their Spinningwork, Vol. I., 



ch. xii. 

 1889. Theridiosoma radiosum, MARX . . Catalogue, p. 551. 



FEMALE: Total length, 2.5 mm.; cephalothorax, 1 mm. long, 9 mm. wide; abdomen, 

 1.5 mm. long, 1.3 mm. wide. These measurements vary somewhat according to the speci- 

 mens, some being larger, some smaller. Count Keyserling, who had thoroughly studied and 

 described Th. gemmosum Cambridge, also knew and described in manuscript the American 



1 Volume I., chapter xli. 



