Clidophorus neglectus.1 LAMELLIBEANCHIATA. 607 



to the narrowly rounded posterior extremity. Anterior end neatly rounded, wider 

 than the posterior. Ventral margin gently convex in the middle, more strongly and 

 almost equally curved at the ends. An obscure umbonal ridge traceable from the 

 beaks three-fourths of the distance to the posterior basal edge. Above it an 

 impressed narrow line, beyond which' the surface descends rapidly to the dorsal 

 margin. Casts of the interior with a narrow, slightly curved, clavicular impression 

 just in front of the beaks, extending but little more than one-third of the distance 

 to the antero-basal margin. Surface of casts with a few obscure growth lines or 

 folds. Point of greatest convexity a little above and behind the center of the shell. 

 In a dorsal view the central half of the outline is very slightly flattened. 



Length, 17.2 mm.; hight, 9.0 mm.; thickness of both valves, 5.8 mm. 



This shell appears to be related to G. cuneatus and C. elongatus, described by Hall 

 from the Silurian rocks of Nova Scotia (Can. Nat. and Geol., vol. 5, pp. 148 and 150, 

 1860). It is, however, specifically distinct, the shape being different and the poste- 

 rior sinus situated higher up and very much less defined. G. planulatus (Conrad) and 

 G. elUfticus Ulrich, also have somewhat different outlines and have the cardinal 

 slopes less abrupt, the whole surface in those species being more uniformly and less 

 convex. 



, Formation and iocaZi%. -Middle Galena near Wykoff, Minnesota, where it is associated with Cteno- 

 donta ihtermedia. 



Clidophorus nbglbctus Hall. 



PLATE XLII, FIGS. 20-25. 



Clidophorus negleotus Hall, 1862. Geol. Sur. Wis., vol. 1, p. 55. (Figured but not described.) 

 Compare Clidophorus (Nuoula) fabula Hall, 1845. Amer. Jpur. Sci. and JVrts, vol. xliii, p. 295. 



Shell varying greatly in size, the smallest observed having a , hight of only 2.5 

 mm., with a length of 5 mm., while in the largest seen (from Graf, Iowa,) these 

 dimensions are respectively 8.5 mm. and 16 mm.; thickness of the latter about 

 6 mm. 



Shell transversly subelliptic, rather strongly convex; ends subequally rounded, 

 the anterior generally a little narrower than the posterior; the outline of the latt'er, 

 however, often exhibits a tendency to become angular just beneath the middle and 

 obliquely subtruncate above; basal and dorsal margin broadly convex. Beaks small, 

 somewhat tumid, placed about one-third of the length of the shell behind the ante- 

 rior extremity. Surface matked by fine concentric lines and several stronger varices 

 of growth; the latter show through the shell so as to be visible on casts of the inte- 

 rior. Hinge plate narrow, not over half the length of the shell, minutely toothed; 

 denticles twenty or more in each valve, three-fourths of the number being posterior 



