Rev. T. Hincks on Reticularia immersa and Halia praetenuis. 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 praetenuis. 

 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 ubietina 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 pubHshed as Halia pratenuis. I could have no doubt tliat 

 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 

 exammation 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 



