156 Trans. Acad. Sci. of St. Louis. 



originally described by Meek and Worthen from Kinderhook, 

 Illinois. Another variety of the species or perhaps a form 

 which should be considered as a distinct species, occurs in the 

 Chonopectus sandstone,* but it always differs from this typi- 

 cal form in having a much wider angle at the beak and is a 

 larger and thicker shell. In the illustrations of the species on 

 Plate XIII. the fine radiating striae which are so characteristic 

 of the shell, are not shown. 



Camarotoechia ? heteropsis (Win.). 



PL XIII. f. 9-13. 



Bhynchonella heteropsis, Bull. U. S. G. S. 153: 533. 



Original description. " Shell small, varying from sectori- 

 form to transversely elliptic, with moderately projecting 

 beak; very young specimens in the shape of a barley-corn. 

 Plications sharp, ranging in number from ten to twenty; of 

 which three generally (sometimes two or four) occupy the 

 sinus of the pedicle valve. This valve has a moderately 

 sharp beak, turned back in an angle of 45° with the plane 

 of the shell, and slit (in the cast) from the apex to the 

 hinge; sinus deep toward the front of the mature shell, want- 

 ing in the young one; the plications on each side of the sinus 

 variable, four in those with two plications in the sinus, six, 

 seven or eight in those with three, and five in those with four, 

 making the whole number of plications ten to nineteen. 

 These lateral plications are bent backwards in approaching 

 the margin. Greatest prominence of pedicle valve near the 

 beak. Brachial valve more ventricose than the pedicle, most 

 prominent at the anterior margin ; mesial fold much less 

 marked than the sinus opposite, consisting of two, three, four 

 or five plications, elevated at their extremities somewhat 

 above the lateral plications, the remotest of which exhibit a 

 strong downward curvature. Beak of this valve concealed 

 beneath that of its fellow." 



" Length 9| mm., breadth 101 mm., thickness of both 

 valves 7 mm. 



Remarks. This species is remarkably variable in form. 

 The commonest variety is a moderately flattened shell with 



* Trans Ac. Sci. St. Louis. 10:72. 



