MACILUS. 



living in a perforation which it has exactly suited to its 

 volume : we are convinced that it has not, nor does it 

 need, the means of enlarging this perforation ; for when 

 quite young, taking up its station in an hollow part of 

 the Madrepore, and increasing itself in size and length as 

 the Madrepore increases around it, it keeps its aperture 

 even with the outer surface of the coral, and thus grows 

 in some instances, to a considerable length. We are 

 informed that this singular testaceous parasite is common 

 in the Coral rocks of the Isle of France, and that its tube 

 sometimes reaches the length of three feet. 



Shell, at its base and in its young state, convolute, 

 ovate, like a snail forming a short spire of four volutions 

 at most : the last volution larger than the rest; and as it 

 increases in age, prolonged into the form of an elongated, 

 irregularly undated, straightish tube, consisting of a 

 solid shelly substance, the greater part of the tube being 

 filled up by the animal as it extends that part of the 

 tube in which it resides, and formed of fibres diverging 

 from the center towards the circumference. Upper part 

 of the tube convex; lower part carinated; sides rather 

 irregularly depressed, with irregular transverse folds or 

 striae, particularly on one side. 



Notwithstanding Lamarck's expressed opinion that 

 the Serpula gigantea of Pallas, appears incontrovertibly to 

 be a species of this Genus; we cannot consider it as such, 

 and in this Our opinion we are confirmed by the very 

 words of Pallas : " Longitudinaliter in rupibus crustisque 

 corcdlinis exporrecti, subflexuosi et per totam longitudi- 

 nem adnati esse solent. Qui Milleporcd alcicorni adcreve- 

 runt, ab ejusdem substantia vulgd penitus incrustati sunt, 

 ut vera eorum forma conspici nequeat.'' 



The only species we are acquainted with are recent ; 

 we have represented two, M, antiquus and M. elliptecus, 

 while we remain unacquainted with the form of the one 

 provisionally named M. Peronii by Lamarck, we can only 

 consider it as a young specimen of M. antiquus. 



