192 BULLETIN 125, UNITED STATES NATIONAL MUSEUM. 



3. On the inner face there are large cavities which, as they are closed by the 

 ectocyst, are indeed hydrostatic. We have not been able to make zoarial sections 

 of such a nature that we can see by what mechanism the sea water enters or departs. 



4. The mandibles of the oral avicularia are identical with those of the avicu- 

 laria of the inner face. 



5. The ovicells observed were marginal and not inserted between the other 

 zooecia, but our specimens are insufhcient to note if this is a constant character. 



6. This species is very close in its dimensions to Mamillopora tuberosa Canu 

 and Bassler, 1919, and differs only in the position of its ovicells and in the con- 

 stancy of its oral avicularia. 



We do not know the inner side of Mamillopora smithi Calvet, 1907. That of 

 Mamillopora cupula Smitt, 1872, has not been figured, but Smitt speaks of "a thin 

 layer of bladders (aborted avicularia) on the back of the zooecia," a description 

 which appears to accord with our observation. Smitt's specimens were deprived 

 of the ectocyst and he was not able to determine the true nature of the avicularia. 



MAMILLOPOKA TUBEROSA Cann and Bassler, 1919. 



Plate 6, figs. 16-19; plate 7, figs. 1-8. 



1919. Stichoporina tuberosa Canu and Bassler, Geology and Paleontology of the West Indies, 

 Bryozoa, Publications of the Carnegie Institution of Washington, no. 291, p. 98, pi. 1, 

 figs. 20-23; pi. 6, figs. 16-19; pi. 7, figs. 1-8. 



Description. — The zoarium is free, conical, hollow, with very thick walls. 

 The peristome is salient, ornamented with small tuberosities; it bears one or two 

 small, elliptical avicularia with bar or denticles. The apertura is elliptical, elongated , 

 hidden at the base of a short peristomie; it is formed of a large, semielliptical anter 

 and of a small, concave poster, separated by two small, salient cardelles. The 

 ovicell is large, somewhat salient, convex; it is hyperstomial and always closed by 

 the operculum. A salient, elliptical avicularium, with two denticles serving as 

 pivot, is placed at the base of each zooecium; it deforms the adjacent peristomes. 

 The inner side is tuberose and bears very large pores arranged in quincunx. On 

 the lower face there are large pores surrounded by very small ones. 



Measurements.-A^evtnr^ ( f'' = O.U mm. Zooecia I ^" =^-^^ ™"^- 



[to =0.10 mm. \lz =0.30 mm. 



Opesium of f Ao = 0.10 mm. a ■ i • fiau = 0.20mm. 



avicularium \ lo =0.06 mm. Aviculanum | ^^^ ^^^^ ^^ 



This is a very elegant species characterized by its peristomial tuberosities. 

 The ancestrula is visible only in the interior of the zoarium; it is covered exteriorly 

 by the first zooecia. All the zooecia are separated from each other by small canals 

 which appear to end in the large, inferior pores. The oral tuberosities are hollow. 

 The pores of the mternal cavity are avicularia, of which the pivot is formed by two 

 denticles; they are analogous with those of the external face (mferior). The 

 internal face (and upper) bears also large cavities which we believe to be hydrostatic 

 cavities; but it must be proved that the ectocyst is resistant enough to confine an 

 equal amount of water. We must suppose also that these cavities are intended to 

 counterbalance the uregularities of calcification and to assure the perfect equilibrium 

 of the zoarial system. 



