100 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Juue 14, 



prominence, agreeing in position and form with the openings on the 

 surface of the branches ; and it occupied only a limited portion of 

 the oblique covering ; or its depth was very much less than the dia- 

 meter of the cavity. These characters prove, to the extent of the 

 fragments examined, that the apertures were not simple tubular ex- 

 tremities, like those in BipTiodictyum gr'acile, TubuHpora, &c., but a 

 distinct structure. In the genus Eucratia * of Lamouroux, the vis- 

 ceral cell swells out from a very narrow base (Lamarck's 2nd Ed. 

 loc. cit.), and the ample interior is closed obliquely in a manner si- 

 milar to that stated above, as shown in M. Milne-Edwards' s figure 

 (op. cit. pi. 8. fig. 1) ; but Prof. J. Reidf has explained that the 

 covering is a membrane, and that it falls off when the polype is dead ; 

 and M. Edwards had previously delineated the operculum by which 

 the small aperture is closed when the tentacula are drawn in {loc. 

 cit.). In Ckisma fvj'cillatum the oblique layer differed not in com- 

 position from the sides of the tubes, and no signs of an operculum 

 were noticed. The walls of the cavities were distinctly foraminated, 

 presenting sometimes even a sieve-like texture ; and they appeared to 

 vary in thickness, in consequence of the intersection including a por- 

 tion of the curvature of the tube (fig. 27) ; but the actual dimension 

 was small. The character of the interval between adjacent walls de- 

 pended seemingly on the position of the cavities, and on circumstances 

 which permitted a certain amount of divergence. Sometimes scarcely 

 a vestige of an interspace was detectable in the middle area of a 

 branch (fig. 26) ; while in other sections less centrally exposed, a 

 small divisional line was occasionally quite distinct ; and in a third case 

 (fig. 27) a fissure prevailed nearly throughout, acquiring in one part 

 great prominence of character. The principal specimen afforded in- 

 stances of each condition. Oblique or transverse slices exhibited also a 

 variable amount of interval. In general the minor fissures were more 

 or less clouded, and at times appeared to be crossed by a bar or fila- 

 ment ; but in the wide spaces (fig. 27) the evidence was in favour 

 of a clear chasm, allowance being made for the interference of tubular 

 walls situated behind the plane of section. A transverse slice (fig. 28), 

 with considerably separated cavities, showed no regular intermediate 

 structure, but the cloudiness in the fissure was generally greater than 

 that within the tube and dissimilar in character. The indications of 

 cross-bars were likewise few and unsatisfactory, as proofs of com- 

 position, when examined with a sufiicient power. This want of 

 connecting structures under certain circumstances suggests the in- 

 ference, that the walls of adjacent cavities were not primarily united, 

 or that there was no interblending of the secreting membranes. 

 Roughly fractured branches afforded sometimes a crenulated surface 



* Lamouroux, Expos. Methodique, p. 8, pi. 65. fig. 10 (Euc. chelafa) : consult 

 also Milne-Edwards, 2ud Edit. Lamarck, torn. ii. p. 188 note, and Ann. des Sc. 

 Nat. 2nd Series, Zool. tome ix. pi. 8. fig. 1. p. 11, Des Crisidies, or Recherches sur 

 les Polypes, Mem. sur les Crisies, &c. 



t Ann, and Mag. Nat. Hist. vol. xvi. p. 392 {Crisia chelata), pi. 12. fig. 10, 

 1845. Consult also Dr. Johnston's Brit. Zoophytes, 2nd. Edit. p. 288-290, 184G- 

 1847. 



