Zoological Illustrations/VolIII-Pl175



leucostoma,

White-mouthed Apple Snail.

—See Pl. 98.


 * A testâ ovatâ, rugosâ, epidermide olivaceo-fuscâ; labio exteriore tenui; aperturâ albâ; umbilico vix clauso.


 * Shell oval, wrinkled; epidermis olive-brown; outer lip thin; aperture white; umbilicus nearly closed.

In prosecuting my illustrations of this genus, I have carefully examined all the specimens in the cabinets of my friends, and have added many to my own. These materials have thrown some additional light on those species which I have already described, and have enabled me to detect several others altogether new. Among the latter is the shell here figured, and which is so rare, that I know but one example of it in this country. Its form is more oval than that of A. rugosa, from which it is likewise distinguished by a very small umbilicus, nearly concealed by the inner lip; the wrinkles are numerous and unequal, the spire pointed, and the aperture milk-white.

Since my remarks on the Planorbis cornu-arietis of Lamarck were published, it has been discovered that the shell is furnished with an operculum: one of these is in the possession of Mr. Sowerby: thus what was a matter of doubt becomes a fact, and affords the only substantial argument for terming it an Ampullaria. On the other hand, its affinities to Planorbis (marked by its discoid, depressed form, and the total absence of the pillar,) remain in no degree impaired. The weight of argument on both sides now appears to be so equal, that it is a matter of no moment whether this shell be placed in the system at the end of the Ampullariæ, or at the commencement of the Planorbes. To the generality of conchologists, the latter collocation would appear the most simple; but, on the whole, I incline more to the propriety of considering it the terminal species of the Ampullariæ, or that which marks their transition (as I before observed) to the Planorbes.