Rev. T. Hincks on Reticularia immersa and Halia prsetenuis. 469 area may be varied, the operculum always retains the circular form." I think the irregularity may be easily explained, when we consider that the animal has to reproduce the operculum in the most rapid manner to replace the lost part, and therefore com-mencing from the centre, it forms only one or one and a half broad whorl, instead of the large number which it gradually deposited. As it has to adapt the operculum to the increased size of the mouth of the shell and of the foot on which it is formed, and the end of the foot of the animal and the circular mouth of the shell not being altered by the abstraction of the operculum, the reproduced operculum is naturally of the form of the pre-vious normally formed one. XLI. — Note on Reticularia immersa and Halia prsetenuis. By the Rev. Thomas Hincks, B.A. In the 'Annals' for February 1855 I described a supposed Polyzoon under the name of Halia prcetenuis. I had never met with the species living, and merely inferred from the character of the cell, &c. that it must be ranked as a Polyzoon, and not as a Hydroid. Mr. Alder, having recently made a careful exami-nation of the common parasite of Sertularia abietina and other zoophytes, which passes as the Reticularia immersa of Professor Wyville Thomson, has informed me that he can detect no dif-ference between this species and the Halia, and that he believes them to be identical. I have now no doubt that his opinion is correct, and that the genus Halia was founded on speci-mens of the zoophyte which Prof. Thomson has described as Reticularia immersa. In characterizing this species, however, he has fallen into a mistake as to the form of the cell, and his figure [vide Annals, Ser. 2. vol. xi. pi. 16) is not an accurate representation of the reality. Deriving my knowledge of Reti-cularia, as I did, from his description and figure, there was nothing to lead me to suspect its identity with the form which I had obtained on mussel-shells from the Dogger Bank, and which I published as Halia prcetenuis. I could have no doubt that the zoophyte of his paper was not the species which I had before me when I constituted the new genus. The cause of this mistake on the part of so able a naturalist may perhaps be found in the difficulty which attaches to the examination of Reticularia in its ordinary state, — the cells being densely packed together and forming a confused mass, amidst which it is no easy matter to trace the form. When the species creeps over shell (as was the case in my specimens) the character
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