ARBACIADiE. 63 



this position would be strengthened by the fact that they are provided with a 

 "neck" and the^ valves have an "articular loop," quite lilie those of the 

 tridentate. 



One of the most striking features of the family, so far as the pedicellarijB 

 are concerned, is found in the structure of the stalk, which is always made 

 up of a bundle of calcareous threads, parallel and unconnected, united only 

 at the top and base, save for their organic covering. When treated with 

 alkali, therefore, and this covering thereby removed, the threads easily 

 separate from each other (Pis. 46, fig. 12 ; 47, fig. ^). The upper end of the 

 stalk is always a dense mass of calcareous tissue of variable size and shape, 

 and it is interesting to note that each species shows a fairly constant, char- 

 acteristic form. The constancy is not so great, however, as to warrant the 

 use of this feature for a specific character (compare Pis. 47, figs. 4, 13, and 18 ; 

 48, fig. 18). 



The tridentate pedicellarifB (Pis. 46, fig. 9 ; 47, fig. 1) are seldom abun- 

 dant, frequently common, often rare, and occasionally wholly wanting. If 

 present, they occur at or below the ambitus, less commonly on the abactinal 

 surface, and very rarely on genital or ocular plates. They show a very 

 extraordinary range of size and form. They are always provided with three, 

 and only three, valves, and these are attached to the stalk by a more or less 

 elongated, muscular "neck." The upper end of the stalk is commonly en- 

 larged and rounded (PI. 49, fig. 25). The valves may be very narrow, 

 (PI. 49, fig. 23), and even compressed (PI. 46, fig. 11), the greatest width 

 less than one-third the length, or they may be very broad and flat (Pis. 48, 

 fig. 15 ; 49, fig. 19), the width two-thirds the length. They range in length 

 from .20 to 2.65 mm., while the stalk is equally variable, sometimes barely 

 equalling the head, often 3-5, rarely 10-15, times as long. The blade usu- 

 ally contains a more or less considerable meshwork of calcareous matter, and 

 often the apophysis is continued as a noticeable ridge, well toward the tip of 

 the valve. Sometimes, however, there is no calcareous meshwork, and 

 often the apophysis simply forks at the base of the blade. In one partic- 

 ular the tridentate pedicellariag of this family are remarkable, and resemble 

 the ophicephalous pedicellariae, and that is in the presence of a distinct 

 " articular loop " on the base of each valve. Unlike the ophicephalous pedi- 

 cellarite, however, the three valves do not show any noticeable individual 

 diversity in any one pedicellaria, the articular loop being of practically the 

 same size on each valve. Individual diversity among tlie tridentate pedicel- 



