ON AN INSTANCE OF COMMENSALISM. 985 



The ectocyst in the surface laj'er of zooecia is membranous. 

 The operculum is 0"06 mm. long and 0"08 mm. wide, and with 

 the rim thickened, especially laterally. In che older layers of 

 zooecia the opesia is filled in, not by membrane, but by a thick 

 punctate, calcareous plate, convex on its lower surface, and 

 further, the zoojcial walls become thickened, so that the zoarium 

 becomes hard and stony. Rosette- plates multiporal ; usually 

 four in each lateral wall ; two, occasionally several, in the 

 distal wall. 



There are no avicularia and no ooecia. 



There need be no hesitation in assigning the Polyzoon to the 

 genus Conopeum Gray * (genotype : lacroixii Audouin f) as 

 amended by Norman J and Canu & Bassler§. 



But the determination of the species has been a more difficult 

 problem. One high authority, to whom the material had been 

 sent, identified the species as Membrunipora tehuelcha d'Orbigny. 

 We were at first inclined to regard the species as a variety of 

 Conopeum lacroixii Audouin, but finally we have come to the 

 conclusion that the species is new to science. We propose to 

 call it Conopeum commensale, sp. n. 



The distinguishing characters of the new species are : the thick 

 zooecial walls, the thick calcareous opesial plate, and the dense 

 stony multilaminate zoarium. Neither the well-known British 

 Con. lacroixii, as described by Busk and Hincks, nor the 

 Mediterranean form, as described and figured by Audouin and 

 Savigny, have any of these characters. If in the British form 

 of G. lacroixii occasionally one layer grows over another, there 

 are no calcareous opesial in-fillings ; and, moreover, " the tri- 

 angular hollows on each side above the aperture " (Hincks, Brit. 

 Mar. Pol. p. 130) have each an opening, covered by membrane, 

 and do not fuse into a single rectangular block. This fusion 

 does, however, take place occasionally in the typical form, figui'ed 

 in the 'Description de I'Egypte,' pi. x. fig. 9. The new species 

 differs widely from M. tehuelcha d'Orbigny. We had the good 

 fortune to find numerous specimens encrusting the alga Macro- 

 cystis pyrifera from the Straits of Magellan and from Valparaiso 

 and the coasts of the Tehuelchan region (the T. being a great 

 tribe of Patagonian Indians). The zooecia (fig. 12) are very 

 different in shape and character, being thin-walled, elongated, 

 rectangular boxes with sharp edges; and the triangular knob 

 and fused rectangular structures of C. commensale Kirkpatrick 

 & Metzelaar are replaced by a pair of rather long, blunt 

 calcareo-chitinous " spines." M. tehtoelcha, in our opinion, is little 



* Gray, List of British Animals.— Part I. Eadiated Animals, pp. 108, 146 (1848). 



t It is held by several authorities that Audouin's " Lao'oixii" is a sjaionym of 

 MXllepora reticulum Linne. 



X Norman, Nat. Hist. East Finmark. Ann. & Mag. Nat. Hist. (7) xi. p. 586 

 (1903). 



§ Canu & Bassler, North American Early Tertiary Bryozoa. U.S. Nat. Mus 

 Bull. 106, text pp. 84, 86 (1920). 



