44 CARBONIFEROUS LAMBLLIBRANCHIATA. 



The anterior border somewhat convex, the lower much so, but the posterior nearly 

 straight. The hinge-line straight, much shorter than the antero-posterior diameter. 

 The umbo of the right valve small and scarcely projecting above the hinge-line ; 

 that of the left much larger, elevated, gibbose, and incurved. Right valve deeply 

 indented by the byssal notch, which forms a triangular pit, and separates a small 

 ear from the rest of the valve. In the left valve, the notch is indicated by a 

 more or less marked depression. 



Interior. — Meek states that the adductor scar is large and subcentral, while the 

 impressions of retractor muscles are several, small, and placed near the beaks, and 

 the hinge is edentulous. 



Exterior. — The surface is adorned with concentric lines of growth, and in some 

 species with radiating more or less scaly ribs, the more distinct on the left valve. 



Observations. — -The genus Eumicrotis was defined by Meek to include shells 

 from the Permian and Upper Carboniferous of North America. He considered 

 that Monotis speluncaria , Schlotheim, sp., and King, from the Permian of England, 

 also belonged to his new genus, and that it had been wrongly referred to Monotis, 

 Bronn. This species, however, has a peculiar posterior lobe separated from the 

 rest of the valve by an oblique sinus ; the left umbo is arched to a greater extent, 

 and the hinge-line is not so pronounced as in Eumicrotis. The type is En microtia 

 Hawni. 



Meek and Hayden say that " the shells embraced in this genus are apparently 

 most nearly allied to Aucella of Keyserling." De Koninck established the genus 

 Butotia to receive shells from the Carboniferous Limestone which were smooth, 

 inequivalve, orbicular or oval, with the left valve more convex and higher than the 

 right; but he did not realise that the genus Eumicrotis, Meek, had been 

 founded for similar shells, and therefore had the right of priority. De Koninck 

 described eleven species, some of which represent different stages of growth of a 

 single species. His figures of E. hemisjrfidsricus, Phill., sp., are not good, and do not 

 show the characters of the anterior and posterior superior angles. 



The most common species is E. hemisphsericus, Phillips, sp., and it shows the 

 peculiar generic characters described by Meek and Hayden. There can be no 

 doubt of the propriety of separating P. hemisphxricus from Pecten, and on account 

 of the inequality of the valves it could not be placed in Posidonomya. The shell 

 has certainly no connection with Aviculopecten. Unfortunately the hinge characters 

 have not yet been observed, but it would appear that there is in the right valve an 

 internal ridge (leaving a groove in casts), which passes from the front of the umbo 

 to the anterior border, forming a curve with the concavity upwards ; while in the 

 left valve is a deep narrow hollow externally between the umbonal margin and the 

 rudimentary anterior ear (PI. VII, fig. 5). Unfortunately this portion of the valve 

 is not complete in the only bivalved example I have yet obtained. 



