ASPIDODIADEMATID.^. 91 



heads are attached to the stalks, which are rarely more than twice as long, 

 by a relatively short " neck," and the njjper end of the stalk is rounded. 

 The valves (PI. 50, figs. 7, 12) range in length from .30 to 2.00 mm., and 

 their greatest width does not exceed .40 of the length. They may be straight 

 and meet for nearly their whole length, or else straight or curved and 

 meet only at the tips. The blade is often compressed ; it seldom contains 

 much ot" a calcareous network, only a few transverse pieces. The stotd 

 tridentate are usually rare and often wanting. They occur chiefly on the 

 abactinal surface, especially close to the genital and ocular plates. The heads 

 are attached to the stalks, which seldom greatly exceed them in length, by 

 a very short neck, and the upper end of the stalk (PI. 50, fig. 15) is much 

 enlarged and slightly flattened. The valves (PI. 50, figs. 1, 6, 11 ) are roughly 

 triangular with a blunt point and are always very deep at the base, but the 

 blade is more shallow and is largely filled by a calcareous network. The 

 valves are from .60 to 2.00 mm. in length, and the breadth at base is from 

 a half to three-fourths of the length. Form b is rarely met with, but is 

 sometimes found abactinally. The valves (PI. 50, fig. 3) are comparatively 

 flat, more oblong than triangular, and the calcareous network is only in the 

 lower part of the blade. The length of the valves is rather more than 

 a millimeter, and the width is about half as much. Form c is quite as rare 

 as b. The valves (PI. 50, fig. 4) are only about .50-60 mm. in length and 

 .20-.25 mm. wide at base, and are remarkable for the presence of an articu- 

 lar loop and the absence of the calcareous network in the blade. 



The ophicephalous pedicellariie are common on all parts of the test, but 

 are chiefly found on the interambulacra. They are remarkable for the very 

 small heads and the presence of three large glands on the stalk. The head 

 rests directly on the enlarged, flattened, or concave end of the stalk, which is 

 6-10 times its length. The valves (PI. 50, figs. 9, H) are only about .20 mm. 

 in length, and their width is a little more than half as much. They may be 

 rounded or pointed at the tip. An articular loop is present, but is small and 

 the three valves do not differ essentially from each other. Doderlein and 

 Mortensen both speak of these pedicellariae as sometimes having four valves, 

 but we have never seen any with more than three.^ 



1 Dr. Mortensen's figures (1904, PI. 4, figs. 10 and 12) represent the valves of ophicephalous pedi- 

 cellariai much flatter than they appear to be to us, and with a rudimentary articular loop or none, 

 a condition we have not observed. We may also mention that his figure 35, pi. 4, cannot possibly 

 be a triphyllous valve, as is stated in the explanation of the plates. This is doubtless a slip 

 of the pen for "tridentate." 



