Synonym: Leptaena tenuistriata
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Map point data provided by iDigBio.
- C6 Sequence (Elkhorn)
- C5 Sequence (Liberty, Waynesville)
- C4 Sequence (Arnheim)
Identification in Hand Sample
- Subrectangular, semicylindrical shape
- 12 or more concentric rugae (ripples) which cross the costae
- Sides are expanded and it is a convex
- Hinge line is straight
- Ventral valve is flat
Jin et al. (1997):
- Remarks: Foeste (1909) described two closely related species, Leptaena richmondensis and L. richmondensis-precursor, from Richmondian rocks at Madison, Indiana, and Arnheim, Ohio, respectively, and distinguished L. precursor from L. richmondensis by its more convex ventral visceral disc, weaker concentric rugae, and less sharp geniculation. The smaller shall, visceral disc and weaker, more numerous concentric rugae of L. richmondensis-precursor are most likely expressions of infraspecific variation within L. richmondensis. Similar variation in shell and disc sizes, convexity of ventral disc, and number and strength of rugae can be found in the Hudson Bay specimens.
- Because most of the Hudson Bay specimens are partly buried in matrix on a single block with only their visceral discs exposed, precise measurements of the overall shell sizes are difficult. The largest specimen is estimated to be 13 mm long and 26 mm wide, approximately the same as the type of L. precursor, but smaller than the type of L. richmondensis. The visceral discs vary from 7 to 11 mm in length and 11 to 17 mm in width. Concentric rugae begin between 2.5 and 5.5 mm from the apex, and are continuous, regularly spaced, and generally stronger toward the disc margins. Numbers of rugae vary from 4 to 7. Costellae are relatively strong, equal in size and spacing, and average 3 per 1 mm. The geniculation of the shall is sharper anteriorly (90° to 110°) than laterally (120° to 140°). The trail may reach a height of 6 mm. The ventral visceral disc is flat to weakly convex, and bears an inconspicuous median fold in some specimens. The dorsal disc is usually flat. The ventral interarea is low, about 1 mm high, weakly to moderately apsacline, with a small delthyrium. No pseudodeltidium was observed.
- Naturally exposed ventral interiors are few in the Hudson Bay collections. The teeth are triangualr and blocky, and the dental plates short and continuous with lateral muscle-bounding ridges (pl. 21, figs. 12-14). Subrhomboidal in outline, the muscle field is bounded by anteriorly tilting, overhanging, blade-like ridges which are mostly strongly developed anterolaterally. The adductor muscle scars are either not well exposed or poorly preserved. In the dorsal interior, the bilobate cardinal process is laterally continuous with the inner socket ridges and lateral bounding ridges of the muscle scars. No chilidium is seen due to either poor preservation or poor exposure of the cardinal area. A small, rounded alveolus is confined laterally by the inner socket ridges and anteriorly separated from the adductor scars by a curved, transverse ridge (pl. 21, figs. 9-11). A low, rounded median ridge originates from this transverse ridge, extends anteriorly for about 2 mm, then tapers to a thin, thread-like ridge, which extends nearly to the anterior margin of the visceral disc. A pair of equally thin, subparallel ridges flanks the median ridge in the anterior part of the disc. The visceral disc is delimited on the inner surface by an accentuated peripheral ridge. Large but abraded papillae are present on the inner surfaces of both valves.
- Shell transversely subquadrate with more or less extended cardinal extremities. Width, 25-40 mm. Length, 0.5 width. This species differs from the well known L. rhomboidalis with which it was formerly identified by the shallower and less numerous wrinkles and broader striations with narrower intervening grooves. The shell is relatively wider and in most specimens the top of the pedicle valve is relatively flat.
- A common and characteristic Richmond species appearing in the Oregonia member of the Arnheim as the variety precursor which is distinguished by the less conspicuous wrinkling, smaller genticulation anteriorly and more convex pedicle valve. In Jefferson County Butts finds it in the Rhynchotrema dentata and Constellaria polystomella zones of the Arnheim.
- Semicylindrical, closely striated; top of the upper (pedicle/ventral) valve with 12 or more concentric rugae, convex; sides expanded. A shell much resembling L. depressa, and about the same size, but ornamented with much closer striae and of thinner substance.
- Semicircular, measuring the disc of the dorsal valve; hinge line extending beyond the width of the shell, the extremities forming small acute ears; dorsal valve with the disc flat, abruptly deflected from the middle; surface marked by about 9 or 10 (12, Sow.) concentric wrinkles, which are crossed by numerous equal crowded striae; cardinal area narrow linear; foramen nearly closed; beak, in many instances, perforated.