112 AMERICAN JOURNAL 



Rhizochilus monodonta, Query. 



1832. Purpura monodonta, Quoy, Voy. d'Astrol. vol. 2, p. 561, 



pi. 37, fig. 9—11. 

 1832. Purpura madreporarum, Sow., Genera of Shells. 

 1835. " monodonta, Kien., Coq.Viv., p. 84, pi. 17, fig. 50. 

 1846. Purpura madreporarum, live., Conch. Icon., sp. 69. 

 1852. Leptoconehus " Gld. Am. Ex. Ex., p. 378, fig. 



483. 

 1854. Purpura madreporarum, Rouss., Voy. au Pol. Sud., p. 



92, pi. 22, fig. 34, 35. 

 1854. Purpura monodonta, Rouss., Voy. an Pol. Sud., pi. 22, 



fig. 32, 33. 



Immature specimens of this species were named monodonta by 

 Quoy. In that state the columella often projects beyond the 

 base, terminating in a sharp pointed tooth. The name should 

 not therefore be rejected, as proposed by Gould and Reeve. As 

 it matures, the canal at base becomes closed and the peritreme 

 continuous, constituting the madreporarum, Sow., well represen- 

 ted in Voy. au Pol. Sud. M. Rousseau separates the two forms. 

 The habit of the animal corresponds to the shell, changing as 

 it matures. When young it is free, and provided with a small 

 short siphon, which scarcely projects beyond the canal. It is 

 sluggish in its movements. As it matures it becomes attached 

 to the coral, on which it lies and adheres with great tenacity, 

 often allowing the foot to be torn away before releasing its 

 hold. The conformation of the lip corresponds exactly with the 

 irregularities of the place of adhesion. Upon removing the 

 animal, scars will be noticed on the coral, more or less worn 

 by the abrasion of the shell, and old specimens will be found to 

 have deposited a shelly base. When removed, the animal is 

 very timid and never wholly expands. It can only partly with- 

 draw behind the columella shelf, leaving a portion of the mantle 

 and the foot exposed. The foot is small, of an oval form, thick 

 and fleshy. It is provided with a small, thin, oblong operculum, 

 having the elements concentric and the nucleus lateral, attached 

 to posterior left side of the foot. The tentacles rapidly taper to 

 a fine point, on which the eyes are sessile a little beyond the 

 middle of their length. The foot is tinged with pale orange, 

 dotted with whiie along the upper margins. The mantle is 

 colorless centrally, tinged with orange along the margins and 

 dotted with white, the dots crowded anteriorly and becoming 

 more and more remote posteriorly. The operculum is of a 

 pinkish violet color. The foot has a well developed duplication 

 in front. 



