436 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



tangentially. The underlying hard rind is formed by the most 

 superficial of the skeletal reticular laminae; the apertures in this 

 are smaller than in the laminae of the interior. 



The fistular processes are hollow. The wall is about 1 mm. thick 

 and consists of a thin dermal membrane, overlying a fairly coarse 

 reticulum of spiculo-fiber. The dermal membrane includes a single 

 layer of tangentially placed megascleres, not closely packed as in the 

 dermal membrane of the body but forming an irregular network, in 

 general unispicular; the 3 or 4 sided meshes about half a spicule 

 length in width ; typically each mesh containing a pore. 



Spicules. — 1. Strongyles (pi. 49, fig. 6, «, 6), cylindrical, smooth, 

 slightly curved, with a typical size of about 360 by 16 u.. From this 

 size the spicules grade down to very small ones, 36 by 10 u,; the 

 smaller sizes are abundant, the intermediate sizes scanty. 



2. Palmate isochelas (pi. 49, fig. 6, c), 20 u, long, in the interior 

 and in the dermal membrane of the fistular processes. 



3. Toxas (pi. 49, fig. 6, d), 60-80 \i long, abundant in the mem- 

 branes of the interior ; the degree of curvature varies somewhat, and 

 the curvature is sometimes asymmetrical. 



Eolotype.—Csit. No. 21340, U.S.N.M. 



Subfamily Hamacanthinae. 



Hamacanthinae Ridley and Dendy, 1887, p. 59. 

 Hamacantha Gray, Topsent 1894c, p. 7. 



Desmacidonidae in which chelas as such are not present; char- 

 acterized by the presence of peculiar microscleres, diancistra. 



Genus HAMACANTHA Gray (1867). 



Vomerula part, O. Schmidt, 1880, p. 82. 



Hamacantha Gray, 1867, p. 538.— Part Vosmaer, 18S5, p. 28; 1S87, p. 352 — 



Topsent, 1894c, p. 7. — Lundbeck, 1902, p. 9.— Topsent, 1904, pp. 215-224. 



Hamacantha Gray plus. Vomerula Schmidt, Ridley and Dendy, 1887, p. 59. 



Habitus varying; massive, erect, incrusting, and papillate forms 

 occur. Skeletal framework consists of polyspicular fibers or tracts, 

 together with scattered spicules, crossing in irregular fashion to 

 form more or less of a network ; spongin absent or present in small 

 amount. Megascleres: generally styles, but sometimes oxeas or a 

 mixture of the two; or styles, tylostyles, and strongyles; in some 

 species megascleres of ectosome, exotyles, differ from those of gen- 

 eral skeleton, having the shape of strongyles or tylostrongyles with 

 roughened outer end. Microscleres: diancistra; in some species, 

 two or even three forms of diancistron, differing greatly in size, may 



