102 INVERTEBRATA OF MASSACHUSETTS. 



other species, and does not attain to a large size, the above dimensions 

 being those of a shell one third longer than the usual size. It is close- 

 ly allied to N. concentrica, Say, which, besides being a fossil species, 

 is not described or figured as having a truncated tip. It is also allied 

 to N. costellaia, Sowerby, " Conch. Illust." f. 8. I have carefully 

 compared our shell vv'ith a specimen of N. niinuta, from the coast of 

 Norway, sent me by Dr. Loven, and can find no difference in the 

 number of teeth, or in any other respect. 



JYote. The following undescribed fossil species, is so closely allied to the pre- 

 ceding, that 1 may, with propriety, introduce a description of it here, though it 

 has not been found in this State. 



NucuLA Jacksonii. 



Shell ovate, convex, inequilateral, posteriorly diminishing to a 

 narrow, ascending beak, truncated at tip, and loith a Jlexure in 

 the margin beneath it ; surface with concentric, elevated lines ; 

 teeth fifteen before, and twenty behind the beaks. 



Figure 65. 



Shell ovate, elongated, ventricose, rather solid ; beaks at the 

 anterior third ; anteriorly rounded ; posteriorly rapidly attenuated 

 so as to form a somewhat ascending beak, truncated at tip ; pos- 

 terior hinge margin straight and sharp, compressed at each side so 

 as to form an area defined by denticulated lines, and smooth ; 

 under the tip is a flexure or shallow indentation of the basal mar- 

 gin, and an elevated ridge runs from the beaks to the lower angle 

 of the tip ; surface covered with fine, crowned, elevated, con- 

 centric lines ; within grooved and irregular, with accumulations of 

 calcareous matter, the most remarkable of which are one under 

 the posterior series of teeth, and a rib going to the middle of the 

 truncated tip ; cartilage pit a narrow cavity, which penetrates 

 through the shell and forms a transverse fissure between the beaks, 

 which are widely separated ; series of teeth strongly curved, 

 fifteen before and twenty behind the beaks, short, very broad, 

 and folded outwardly. Length 1 inch, height ^^ inch, breadth 

 ■| inch. 



Found at the land-slip at Pride's Bridge, Presumpscot River, 

 Westbrook, Maine, September, 1837 ; and also by Dr. C. T. 

 Jackson at Augusta, while engaged in the geological survey of 

 Maine, in honor of whom I have named it. 



