Maryland Geological Survey 267 



straight, usually a little concave on the anterior third; posterior margin 

 abrupth' rounded below and more gently curving above; anterior end 

 produced beyond the beaks, abruptly rounded, limited by a broad de- 

 pression extending from the beak to about the anterior third of the basal 

 margin; cardinal margin oblique in the prevailing forms, moderately 

 arcuate. Valves moderately convex, gibbous along the umbonal slope; 

 the point of greatest convexity is about the anterior third of the length 

 of the shell; hinge-line extending half or more than half the length of the 

 shell; beaks subanterior, small, appressed and directed forward; a sub- 

 angular elevation of the umbonal region, usually dying out about the 

 middle of the length of the shell. Test comparatively thick, strongly 

 ornamented by regular concentric, rounded or subangular striae, which 

 become lamellose and coalescing on the anterior end of the valves, where 

 they are less prominent; anterior muscular impression strong, striated, 

 situated just within the anterior margin; posterior impression large and 

 shallow." Hall, 1885. 



Several specimens which are referred to this species were found in the 

 coarser arenaceous shales of the Hamilton in Maryland. Part of the 

 specimens are internal impressions ; but others are more or less exfoliated 

 ones of the shell. On part of them the concentric striae are somewhat 

 coarser than on most of the figured New York specimens ; but others appear 

 to agree with those from that state. This species is closely related to M. 

 mytiloides (Con.) ; but these specimens are apparently differentiated from 

 it by their smaller size, more arcuate form, more gibbous umbonal region 

 and the strong, regular, concentric striae. Hall identified this species 

 from the Hamilton sliales, near Cumberland (Pal. N". Y., vol. v, pt. i, 

 Lamellibranchiata ii, p. 276) and figured the interior of two specimens 

 from Cumberland (ibid., pi. xxxvi, figs. 14, 16). One of the more doubt- 

 ful specimens was submitted to Dr. Grabau who agi-eed in referring it to 

 M. concentrica; as well as another specimen from Avestern Maryland the 

 exact locality of which is unknown. 



Length, 40-60 mm. ; height, 25-37 mm. 



Occurrence. — Eomney Formation, Hamilton Member. East bank 

 Evitts Creek below Wolfe Mill; Williams Road 3^ miles southeast of 



