SlPATANGINA. 97 



esting point that in several genera, ophicephalous pedicellariae are present 

 often abundantly in young specimens, and are totally lacking in adults. This 

 fact is probably of phyllogenetic significance. When present, ophicephalous pedi- 

 cellariae are more generally found on the posterior part of the test especially 

 in the ambulacra, and more commonly on the oral side but they may be found 

 in some species almost anywhere; they always have three valves. The tri- 

 dentate are the most generally present, the most extraordinarily diversified and 

 the most uniformly distributed of the pedicellariae, but they may be quite 

 wanting and not infrequently seem to be uncommon or hard to find. Of course, 

 they typically have three valves but a bidentate form is known in at least one 

 species, and forms with 4, 5, 6, 7 or even 8 valves are not uncommon in some 

 genera. These multidentate forms are of considerable interest and may afford 

 a good specific character. The typical tridentate may be large or small, and 

 stout or slender; the valves may be broad and flat or narrow and greatly com- 

 pressed; they may be straight or curved, in contact throughout or only at 

 tips, and the margins may be smooth, finely serrate, coarsely serrate, dentate, 

 coarsely toothed, or even lobed. Tridentate pedicellariae occur almost anywhere, 

 but around the periproct or the peristome or along the sides of the ventral 

 ambulacra are the favorite locations. The rostrate pedicellariae are a modi- 

 fied form of tridentate, usually quite easily distinguishable by their stout, 

 curved valves meeting only at tip where they are usually more or less widened. 

 Oftentimes it is not easy to draw a line between stout tridentate and rostrate 

 pedicellariae; on the other hand it is sometimes hard to decide whether a given 

 form should be called rostrate or globiferous. The rostrate pedicellariae always 

 have three valves and their distribution is like that of the tridentate; they are 

 generally present. The triphyllous pedicellariae are commonly present, but are 

 often difficult to find, owing to their very small size. The valves are flat and 

 leaf-like, but the relative length and breadth of the valves shows some diversity, 

 and they occasionally afford a specific character. The sphaeridia occur in all 

 the genera probably; at any rate, I know of none in which they are lacking. 

 They are usually not imbedded in the test nor even placed in pits but lie on the 

 surface of the ambulacral plates, at least those near the mouth. In some genera, 

 however, they are placed in pits of greater or less depth and in a few cases they 

 are really imbedded in the test as in clypeastroids. Neither the form, number, 

 nor position of the sphaeridia seem to furnish any character of taxonomic impor- 

 tance. The calcareous particles in the tube-fee! afford characters of no impor- 

 tance for systematic purposes. In the ordinary tube-feet they are often quite 



