SILICIOUS AND HORNY SPONGES WILSON. 301 



forms, of course, occur, the clads of some of them lying in the 

 outermost part of the choanosome. 



(2) Oxeas; equiended, smooth, tapering gradually toward each 

 end; intermingled with the triaenes of the body and cloacal tube; 

 3.7—1.7 mm. long by 50-70 [x wide. 



(3) Orthodiaenes and orthomonaenes, chief megascleres in the 

 walls of cloacal tube. The characteristic spicule (pi. 45, fig. 15b) is 

 a diaene that is nearly a monaene ; rhabdome 3.7-4.4 mm. long, 35-50 

 p. thick; developed clad long, reaching 1,750 pt. in length, about 35 [/. 

 thick, frequently somewhat sinuous; vestigial clad very short, often 

 more or less curved or bent. The vestigial clad reaches a consider- 

 able development in some spicules, remaining shorter, however, than 

 the other clad. In some of the diaenes (pi. 45, fig. 15a) in the basal 

 region of the cloacal tube the long clad is dichotomous, the secondary 

 clads usually unequal in length. 



These modified triaenes of the tube walls are closely and elegantly 

 combined, forming a firm skeleton, the rhabdome playing the part 

 of a longitudinal element, the clads that of circular elements. 



(4) Strongylasters (pi. 45, fig. 12, a-d), not strictly euasters, but 

 representing a step from the euaster toward the streptaster type. 

 Densely abundant in the dermal membrane of the body and cloacal 

 tube, forming a crust; also abundant in the ectosome, and extending 

 into outer part of choanosome. The presence of an axis in (all of?) 

 these spicules taken in connection with the fact that the genus is 

 obviously related to the Stellettidae rather than to the Theneidae, 

 suggests (a) that a series of changes may convert a euaster into a 

 true streptaster and (b) that this series of changes may be begun 

 independently in different though related sponges. 



The spicule is variable. In the common type (fig. 12«) there is a 

 short curved axis bearing about 10 rays which are strongylate and 

 larger terminally; total length of spicule 10 \>.. Forms differing 

 from the above in having fewer rays, 4-5, also occur (fig. 12 6, c). 

 The spicule frequently appears as a euaster (fig. 12d), but this 

 appearance in many cases (always?) is probably due to the spicule 

 being seen in end view. 



(5) Oxyaster (pi. 45, fig. 12e). Present in both ectosome and 

 choanosome. Total diameter 14-20 pi; centrum of considerable size; 

 rays long and pointed; often about 9 rays round equator when the 

 spicule is seen more or less in optical section. 



(6) Trichodragmas, about 20 by 12 pi; common in ectosome. The 

 dragma, or bundle, is cylindrical ; the component rhaphides very 

 fine. 



Tlolotype.— €at. No. 21302, U.S.X.M. 



A related species, T. columnifer Stewart is recorded from the 

 Philippines (Sollas, 1888, p. 190). In this species the soma! triaenes 



