71 



mellaj accompanied by a copious secretion of calcareous matter from the 

 mantle. 



It may be observed that the growth of the Magilus presents a sort of 

 negative analogy with that of the Nautilus ; the animal of each advancing 

 in the outer porch of its shell by the aid of a relaxing muscle ; and their 

 mode of operation is curiously adapted to the difference in their circumstances 

 of habitation. The former, in order to sustain the increasing pressure 

 of the coralj fills up the vacated portion of its shell with solid matter, the 

 latter, having a different medium to contend with, chambers in its shell 

 by a succession of light partitions to give it buoyancy ; if the Magihis pur- 

 sued its course by the deposit of light transverse partitions, after the manner 

 of the Nautilus, the increasing growth of the coral would probably crush it 

 (unless the effect be nullified, as in the case of the Pholades, by a circu- 

 lating current of solvents), and if the Nautilus advanced in its shell by 

 solidifying the vacated portion, after the manner of the Magiliis, it would 

 produce an incumbrance incompatible with its locomotive faculties. 



Like all mollusks inhabiting coral, the Magilus is almost devoid of 

 colouring matter ; the shell is of a transparent alabaster wliite, whilst the 

 edge of the mantle of the animal is tinged with violet. I am not aware 

 that this mollusk has been found in any other locality than the shores of 

 the Eed Sea, or in any other species of coral than the Meandrina plirygia. 



Figure. 



Magilus antiquus. PI. 6. Fig. 25. Showing its vermiform structure, 

 keel, and siphonated aperture. — From the cabinet of H. Cuming 

 Esq ; collected hy Br. Riippell, at the Red Sea. 



Genus 14. TRICHCTROPIS, Broderip. 



Animal ; unJcnoivn. 



Shell ; thin, turhinatcd, umhlicated; and sligMly cancdiculated at 

 the base, covered with a strong horny epidermis, sometimes fur- 

 nished with spiral roios of fine bristles, spire angularly turreted ; 

 aperture triangularly ovate ; operculum thin, laminated. 



Although a specimen of this remarkable shell was collected with the soft 

 parts by Captain Belcher, I am not aware that the animal has ever been 

 described ; its true relation in the molluscous system still remains to be 



