118 SUPPLEiMENT TO THE 



The dorsal side of the spirals is flattened, wliilst the ventral side is somewhat ventricose. 

 The spirals are narrow anteriorly, but broader on the posterior side ; and the principal 

 coils on the hinder side are slightly notched or indented, the notch or indentation 

 being in the direction of the end of the spiral, and occupying in most cases the whole 

 breadth of the posterior border. This edge of the spirals, including, of course, the 

 notch referred to, slopes slightly downwards on the ventral side towards the anterior 

 margin ; and arising from this the notch is partly seen in the ventral aspect of the spirals. 

 The slope of the posterior border of the spirals also accounts for the upper part of the 

 coils on the ventral side being more depressed and less oval than the corresponding 

 part of the coils on the dorsal side. As already observed, the discovery of this new 

 genus, and of all its internal characters, is due to the indefatigable exertions and persevering 

 researches of the Rev. Norman Glass, who, after many trials and immense trouble and 

 patience, was enabled gradually to expose the spiral coils, their mode of attachment to the 

 hinge-plate, and their connecting process in the most unmistakable manner. I there- 

 fore had (in 1881) much pleasure in naming this genus after its discoverer. 



The shells belonging to the genus Glassia are not very rare in some places ; but, 

 although comparatively not uncommon in the Buildwas Shales, they occur very often in 

 an incomplete and crushed condition. Good examples suitable for Mr. Glass's operations 

 did not occur in any great number. 



As some uncertainty prevailed in my mind whether the specimens of Glassia here 

 described from the Buildwas bed were referable to Sowerby's species, I borrowed from 

 the Geological Society's Museum Sowerby's type specimens of both Atrypa ohovata and 

 A. compressa (see my ' Sil. Mon.,' PI. XII, figs. 16 and ID), also a series of specimens 

 so named (from the Wenlock Shales of Woolhope) in the Museum of Practical Geology, in 

 addition to a large and illustrative series of twenty specimens from the Wenlock Shales in 

 a railway-cutting near Dudley, lent to me by Mr. John Gray, of Hagley. 



After a very careful study of all this material it appeared to me that the so called 

 Atrypa obovata and A. covipressa, of Sowerby, are nothing more than modifications, in 

 shape, of a single species, and referable to my new genus. 



The muscular and vascular impressions in the ventral valve of Glassia olovafa are 

 shown in PI. XIII, fig. 5, of my * Silurian Monograph.' On the internal cast of the dorsal 

 valve of a specimen from Buildwas can be seen a longitudinal indented line extending 

 from the umbo to half the length of the shell and denoting the presence of a short ridge 

 or septum in that valve. 



Having received, through the kindness of Prof. Lindstrom, of Stockholm, a number 

 of small shells from the Wenlock formation of the Island of Gothland, which had all 

 the appearance of young individuals of Glassia ohovata, or of a closely allied species, I 

 asked the Rev. Norman Glass to develop a few^ of them so as to ascertain to what genus 

 they belong. They proved to be all perfectly characterised specimens of Glassia. 



The genus Glassia belongs to the family Atrypidce although it differs materially from 



