Potamolithus rushii is a species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 of freshwater snail

Freshwater snail

A freshwater snail is one kind of freshwater mollusc, the other kind being freshwater clams and mussels, i.e. freshwater bivalves. Specifically a freshwater snail is a gastropod that lives in a watery non-marine habitat. The majority of freshwater gastropods have a shell, with very few exceptions....

 with an operculum

Operculum (gastropod)

The operculum, meaning little lid, is a corneous or calcareous anatomical structure which exists in many groups of sea snails and freshwater snails, and also in a few groups of land snails...

, an aquatic

Aquatic animal

An aquatic animal is an animal, either vertebrate or invertebrate, which lives in water for most or all of its life. It may breathe air or extract its oxygen from that dissolved in water through specialised organs called gills, or directly through its skin. Natural environments and the animals that...

 gastropod mollusk in the family

Family (biology)

In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

 Lithoglyphidae

Lithoglyphidae

Lithoglyphidae is a family of small freshwater snails with gills and an operculum, aquatic gastropod mollusks.This family is in the superfamily Rissooidea and in the clade Littorinimorpha .- Taxonomy :Taylor , Ponder & Warén and Kabat & Hershler considered this taxon as a subfamily...

.

Potamolithus rushii is the type species

Type species

In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...

 of the genus Potamolithus.

The specific name rushii is in honor of Dr. William H. Rush (18??-1918), who collected this species.

Distribution

The distribution of Potamolithus rushii includes the Uruguay River

Uruguay River

The Uruguay River is a river in South America. It flows from north to south and makes boundary with Brazil, Argentina, and Uruguay, separating some of the Argentine provinces of the Mesopotamia from the other two countries...

 near Paysandú

Paysandú

-Transportation:The city is served by Tydeo Larre Borges International Airport.-Climate:Paysandú has a humid subtropical climate, described by the Köppen climate classification as Cfa. Summers are warm to hot and winters are cool, with the occurrence of frosts and fog...

 (the type locality), Uruguay and Argentina.

Description

Potamolithus rushii was originally described by the American malacologist Henry Augustus Pilsbry

Henry Augustus Pilsbry

Henry Augustus Pilsbry was an American biologist, malacologist and carcinologist, among other areas of study. He was a dominant presence in many fields of invertebrate taxonomy for the better part of a century...

 in 1896. Pilsbry's original text reads as follows:
The shell of Potamolithus rushii is imperforate, wider than high, biconvex, very solid and strong. It is light green in color. The last half of the last whorl is dusky green. The keels are rather bright green. The early whorls are being dark reddish-brown. The surface is somewhat glossy, with faint, fine growth-lines and barely perceptible spiral lines. The spire

Spire (mollusc)

A spire is a descriptive term for part of the coiled shell of mollusks. The word is a convenient aid in describing shells, but it does not refer to a very precise part of shell anatomy: the spire consists of all of the whorls except for the body whorl...

 is convex, the apex

Apex (mollusc)

Apex is an anatomical term for the tip of the mollusc shell of a gastropod, scaphopod, or cephalopod mollusc.-Gastropods:The word "apex" is most often used to mean the tip of the spire of the shell of a gastropod...

 is obtuse. The shell has 4 whorls

Whorl (mollusc)

A whorl is a single, complete 360° revolution or turn in the spiral growth of a mollusc shell. A spiral configuration of the shell is found in of numerous gastropods, but it is also found in shelled cephalopods including Nautilus, Spirula and the large extinct subclass of cephalopods known as the...

, but the first whorl is eroded, leaving a pit, in all the adult shells seen. The whorls are convex, with seam-like sutures. In the latter part of the penultimate whorl the peripheral keel is usually visible at the suture. The last whorl has a very strong peripheral keel, the surface is being concave above and below it. Above the concavity the upper surface is convex, the convexity rising into a hump on the back, then disappearing, the last fourth of the whorl being flat. The base has a thick and prominent keel, defining a concave yellowish columellar area. The outer lip has a high, narrow varix at the edge. The aperture

Aperture (mollusc)

The aperture is an opening in certain kinds of mollusc shells: it is the main opening of the shell, where part of the body of the animal emerges for locomotion, feeding, etc....

 is very oblique, short-ovate, nearly circular, with a continuous, black-edged margin. The oblique columella is very broad, with a gutter or concavity near to and parallel with the inner margin. There is some variation in the degree of depression of the whole shell, the amplitude of the columellar area and in the prominence of the hump on the back, which is sometimes almost suppressed.

The width of the shell is 6.3 mm. The heoght of the shell is 4.3-5.1 mm.


The relationship between Potamolithus rushii and Potamolithus iheringi is exceedingly interesting. The two species are similar in general color-scheme, in the varix, absence of more rapid descent of the suture towards the mouth, etc., but are totally diverse in contour, the one being carinate, the other smooth and naticoid. Yet it is significant that while Potamolithus iheringi has no trace of a peripheral keel, the green band occupies the same position as that coloring the keel in Potamolithus rushii.

Life cycle

The youngest specimens seen by Pilsbry had three whorls and a diameter of 3 mm. They had the depressed contour of adults and were strongly carinate peripherally, but the carina is distinctly weaker in front of the mouth, apparently indicating that it begins when the shell has nearly two whorls and a diameter of about a millimeter. At the 3 mm stage, the columella is very broad, semicircular, with a deep excavation and rod-like inner border (see image on the right). Very late in the neanic stage the basal keel appears, the shell then being about 5 mm in diameter; the columellar area is very narrow, at first linear. The rib or convexity of the upper surface is also of late appearance, these structures belong to the third neanic substage, the second, or unicarinate, substage thus occupying the greater part of the neanic stage. The discontinuation of the upper ridge or hump initiates the ephebic substage. The marginal varix and the absence of any tendency of the last whorl to descend or loosen its coil anteriorly, show that this species is at its acme. It has none of the stigmata of senility which are so manifest in Potamolithus microthauma, Potamolithus hidalgoi, etc.