LOWER SILURIAN LAMELLIBRANCHS. *»T:i 



Hinge plate thick and wide, almost flat, with a ridge forming the 

 lower edge in the posterior part; cardinal teeth strong, very oblique, two 

 in each valve, the second often small and obscure. Anterior muscular 

 scar deep, acuminate-ovate, pointed below where the well marked pallial 

 line joins it. Posterior scar very faint, broadly ovate, situated near the 

 post-cardinal angle. In casts of the interior the beaks are prominent 

 and strongly compressed, while a deep and wide sulcus, extending from 

 the beaks nearly to the center of the base, produces an obtuse umbonal 

 ridge of which no sign is apparent on the exterior of the shell. Another 

 but less deep and shorter furrow occupies the space between the long 

 sulcus and the anterior muscular scar and pallial line. Within the latter 

 space the casts often exhibit a series of four or five rounded elevations. 



A large testiferous specimen has the following dimensions: Length 

 39 mm.; greatest oblique length 41 mm.; greatest posterior heighth 83 

 mm.; from post-cardinal angle to antero-basal margin 35 mm.; greatest 

 thickness 20 mm. 



Formation and Locality. — Near the top of the Cincinnati group, Ox- 

 ford, Ohio, and Richmond, Indiana. 



Ischyrodonta decipicns, n. sp. 



Plate 54. Figs. 16-19. 



Shell scarcely attaining medium size, moderately convex, the beaks 

 small, the umbonal ridge distinguishable though not strong, the outline 

 almost regularly oval excepting that the cardinal region is produced and 

 angular at the posterior extremity. Surface marked with numerous, 

 strong and more or less irregular concentric lines of growth. Cardinal 

 teeth nearly horizontal, three in the right valve, the central tooth much 

 the largest, and two in the left valve. Posterior to the cardinal teeth the 

 hinge plate bears three or four slightly diverging slender ridges, which 

 served as supports for the internal ligament. Muscular impressions sub- 

 equal, strongly marked, the anterior one especially; pallial line distinct. 

 Internal umbonal ridge undefined so that the surface of casts of the in- 

 terior is comparatively even. 



This species is founded upon an excellent series of specimens, most 

 of them recently obtained from Prof. Joseph Moore and Mr. John 

 Misener of Richmond, Indiana. One specimen I had for at least ten 

 years believed to belong to the similar Orionella hainesi Miller, so., and 

 it is the liklihood of confusion with that species that has suggested 

 the name decipiens. A careful comparison however brings out a 

 number of differences that will appear very obvious to the student after 

 he has once made himself familiar with them. First, the surface mark- 

 ings are much coarser in the Ischyrodonta; next, the outline will be found 

 to be not strictly the same; then the Ortonella has a well developed 



43 G. O. 



