NATURAL SCIENCES OF PHILADELPHIA. 107 



ALIORISMA WINCHELLI, Meek. 



Shell of about medium size, elongate subelliptic, the length 

 being equal to about three and a half times the height, moderately 

 convex ; posterior extremity a little gaping, obliquely subtrun- 

 cated above and narrowly rounded below the middle ; anterior 

 extremity extremely short, closed, concave in outline obliquely 

 forward and downward from the beaks above, to the lower 

 end of tlie lunule, where it is subangular, and from this point 

 rounding off obliquely into the base ; ventral margin forming a 

 broad, gentle curve, but generally somewhat straightened, and 

 sometimes faintly sinuous near the middle ; dorsal margin nearly 

 straight, or a little concave in outline, and showing the usual in- 

 flection, which forms a lanceolate escutcheon with a slight ridge 

 on each side, from the beaks to the posterior extremity of the 

 hinge, which equals about three-fourths the entire length of the 

 valves ; beaks much depressed, very oblique, incurved, and 

 located onl}- about one-fourteenth the entire length of the shell 

 from the anterior extremity ; posterior umbonal slopes merely 

 somewhat prominently^ rounded ; anterior umbonal slopes gene- 

 rally subangular near the beaks, and sometimes this prominence 

 is obscurely continued as a faint rounded ridge obliquely back- 

 ward and downward to a point a little in advance of the middle 

 of the base. Surface ornamented with concentric lines and ridges 

 of growth, that generally assume the character of little regular 

 wrinkles on the umbones. Lunule small, rather deep, moderately 

 well defined, and obovate in form. 



Length of a mature specimen, 1.1 i inches; height to middle of 

 dorsal side, 0.83 inch; do. to horizon of beaks, 0.87 inch; con- 

 vexity, 0.70 inch ; length of hinge line, 1.17 inches. 



This is a very neat, symmetrical species, often found in an excel- 

 lent state of preservation as casts of the exterior showing per- 

 fectly the form and surface-markings, excepting the fine granula- 

 tions usually, if not always, existing in species of this genus. 

 Like other species of the group, it varies more or less in form, 

 some individuals being proportionally shorter and higher than 

 others. In size and general appearance it closely resembles A. 

 clavata of McChesney, from the Chester group. It does not re- 

 semble the particular variety of that shell, however, figured by 

 Prof. McChesney, so nearly as it does what I have always believed 

 1871.] 



