Incatina, DeVries, 2019

DeVries, Thomas J., 2019, Early Paleogene brackish-water molluscs from the Caballas Formation of the East Pisco Basin (Southern Peru), Journal of Natural History 53 (25), pp. 1533-1584 : 1548-1549

Genus Incatina gen. nov.

Type species: Incatina collinsae sp. nov., by monotypy. Early Eocene, southern Peru. Species included: Incatina collinsae sp. nov.

Diagnosis

Shell small, compressed, higher than long; posterior truncate; beak medial. Sculpture of right valve composed of evenly spaced and equally sized commarginal ribs; sculpture on left valve may consist of broad triangular ribs with subsidiary commarginal ribs. Lunule commissure strongly sinusoidal; escutcheon narrow.

Description

Shell less than 20 mm long, compressed-convex, higher than long (H:L = 1.07). Beaks medial, pointed, orthocline. Dorsal margins steeply inclined; anterior margin sharply rounded; posterior margin truncate; ventral margin evenly rounded. Lunule moderately wide, extends 40% of anterodorsal margin length. Escutcheon deep, narrow, extends 60% of posterodorsal margin length. Anterior and posterior ends steeply rounded; posterior and anterior umbonal ridges absent. Sculpture of sharply rounded regularly spaced, equally sized commarginal ribs from umbo to ventral margin and extending to anterior and posterior margins; interspaces nearly as wide. Commarginal ribs, less than full length, occasionally inserted or truncated. Right valve in some specimens with eight to nine bluntly triangular radial ribs. Lunule commissure sinuous, lunule asymmetric from left to right valve. Hinge and teeth not exposed.

Remarks

Many astartid crassatelloids are as high as or higher than long and have regular and equally spaced low commarginal ribs, as do specimens from the Caballas Formation, but none have both a sinuous lunule commissure and truncate posterior margin. Most crassatellid species of the Australasian Eucrassatella Iredale, 1924 and the American Hybolophus Stewart, 1930 have truncate or produced posterior margins and sinuous lunule commissures, but such species are substantially longer than high, larger, and more convex, excepting such species as the Quaternary Californian Hybolophus fluctuata (Carpenter, 1864) .

The most remarkable feature of Incatina gen. nov. is the presence of a different sculpture on valves of the same individual: radial on the left, with infrequent commarginal ribs, and commarginal on the right, with no radial sculpture. Such contrasting sculptural plans are a certainty for one specimen ( UWBM 107638) and possible for a second specimen with a poorly preserved left valve ( UWBM 107637). Contrasting sculptural plans are definitely not present on a third specimen ( UWBM 107639). Extant crassatelloids do exhibit unusual sculpture, albeit on both valves. Commarginal ribs of the crassatellid Crassatina Kobelt, 1881 irregularly drift in a posteroventral direction; inverted divaricate ribs ornament the West African and Mediterranean astartid, Gonilia Stoliczka, 1871 ; and sashlike acentric ribs obliquely cross commarginal ribs on the West African and southern European astartid, Digitaria Wood, 1853 .

Etymology

’ Inca ’, referencing the Inca civilisation of Peru.