|Basic info||Taxonomic history||Classification||Relationships|
|Morphology||Ecology and taphonomy||External Literature Search||Age range and collections|
Rhynchonellata - Orthida - Hesperorthidae
If no rank is listed, the taxon is considered an unranked clade in modern classifications. Ranks may be repeated or presented in the wrong order because authors working on different parts of the classification may disagree about how to rank taxa.
|R. J. Ross 1959||Exterior.-The outline is variable, being subsemicircular, subsemiellipitical or subrectangular. The brachial valve is gently convex with a faint median sulcus that fades out anteriorly. The pedicle valve is gently convex at the umbo, flattening anteriorly; in old specimens it is slightly resupinate toward the margin. The dorsal area is anacline and short, while the pedicle interarea is apsacline, plane to very slightly curved, about a third as long as wide.
A few specimens of brachial valves show thin uplifted edges on the inside of the notothyrium (pl. 54, fig. 21). These and a single small specimen with an arched plate in place indicate that there was a delicate chilidium with the general shape of those in H esperorthis (compare pl. 54, fig. 35 with Schuchert and Cooper, 1932, pl. 4, figs. 17, 18, 29). The pseudodeltidium is essentially as in H esperm·this; a few specimens have an arched rather than flat plate (pl. 54, figs. 14, 16) and a very few have the pseudodeltidium isolated by conjoining adventitious material added to the sides of the delthyrium. In these last the pseudodeltidium seems to have been built· forward beneath the adventitious material (pl. 54, figs. 23, 24).
Specimens in which the edges of the delthyrium are raised in the same manner as the edges of the notothyria mentioned above are fairly numerous; these originally led me to surmise that an arched deltidium must have been present. Addition of material along the edges of the delthyrium is common in the Hesperorthinae (Schuchert and Cooper, 1932, p. 85).
The ribbing is more highly variable than other features of the shells. There is a definite tendency for external ribbing to become obscure after shells have reached a length of 15 mm. Some are paucicostate, others multicostate. Counts made at an arbitrary . median length of 18 mm from the umbo of the pedicle valve indicate that simple costae average 7 in 10 mm of width, and may be expected statistically to number as low as 4 and as high as 10. For shells in which smaller ribs are intercalated between the costae the total count at 18 mm from the pedicle beak may be expected to range as high as 14 in 10 mm of width.
Interior of the ventral valve.-The description of the pedicle interior follows almost exactly that given by Schuchert and Cooper (1932, p. 88) for the genus Dolerorthis, but most of the differences which they considered interspecific are here clearly variations among individuals of the one species. The distinctness with which the median ridge and two pallial trunks are developed is highly variable; this cannot be attributed entirely to mode of preservation. (Compare pl. 54, figs. 4, 14, 19, 24.)