Author Description

This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1914 edition. Excerpt:...its surface is marked by concentric wrinkles parallel with its anterior margin which are usually discontinuous along the median line. Remarks.-The genus Dielasma was established by King with Tcrebratula dongatus Schl. as genotype, and although he defined the genus primarily upon the presence of prominent dental lamellae in the pedicle valve and on the form of the loop, his illustrations of the internal casts of the species under the name Epithyris elangata1 show that the crural plates are separate from the socket walls, one of the most essential features of Dielasma as here defined. Davidson2 gives illustrations of the same species which 1 Mon. Permian Foss. Eng., pl. 6, figs. 37, 41. (1850.) 2 Brit. Foss. Brach., vol. 2, Permian, pl. 1, figs. 18, 20. (1857.) exhibit all the essential generic characters of Dielasma most perfectly. The interpretation of the genus by Hall and Clarke1 is identical with that here given, but those authors included certain species in the genus without sufficient investigation of their internal characters, which are really fundamentally different; it has in fact been the usual custom among American workers, since the publication of Hall and Clarke's work, to refer all Mississippian terebratuloid shells to the genus Dielasma. In specimens preserved in the condition of internal casts the generic characters of Dielasma are always very obvious, the position of crural lamellae, separate from the socket plates being indicated by a pair of slits diverging from the beak of the brachial valve; when the transverse muscle-bearing plate is attached along its mesial line a second pair of diverging slits are present between those formed by the crural lamellae and the finger-like casts of the slender cavities beneath the...