238 NEW FOSSIL SHELLS 



FAMILY MACTRID.E. 



GENUS ALIGENA, (Nosis.) 



Testa aquivalvis? subaquilatcralis, postice et antice clausa; cardo dente cardinali uno, sulco sub natibus longo, minime 

 profundo. 



Shell equivalve? sub-equilateral, closed posteriorly and anteriorly; hinge with one cardinal tooth, and a long shal- 

 low sulcation under the beaks. 



Remarks. The cardinal tooth is, in general, rather small. The sulcus appears to 

 have received the ligament. It commences at the beak, and runs obliquely past the 

 dorsal margin into the cavity under the beak. As I possess only odd valves of both the 

 following species, I am unable to determine whether the shell is equivalve or not. 



I have in vain endeavoured to find some genus, already described, which might receive 

 these little shells, but their peculiar characters in having one cardinal tooth, and a liga- 

 mentiferous sulcation, descending into the shell, distinguish them from other genera. 

 Their place in the systems is, most probably, in the family MactridaB, between Erycina 

 and Ungulina. 



I have called the genus, Aligena, one of the surnames of Venus, from its resemblance 

 to Erycina, also an appellation of that goddess. 



A. STRIATA, PL. 34. FIG. 13. 



A. testa transversd, sub-elliptica, postice et antice rotundatd, inflatd, tenui, striatd; striis concentricis, prope marginem 

 basalem maximis; clivo umboniali elevato, sub-angulato; margine basali curvato; margine dorsali media angulato, postice et 

 antice curvato; natibus prominentibus^ incurvis, sub-Iambus; dente cardinali minima; sulco longo, pene transverso; margine 

 interno integro. 



Shell transverse, sub-elliptical, rounded anteriorly and posteriorly, inflated, thin, striate ; striae concentric, very 

 large near the basal margin; umbonial slope elevated, somewhat angulate; basal margin curved; dorsal margin angu- 

 lar in the middle, curved posteriorly and anteriorly; beaks prominent, incurved, nearly smooth; cardinal tooth very 

 small; sulcation long, nearly transverse; margin entire within. 



Diam. .10. Length .19. Breadth .21 of an inch. 



Remarks. The concentric stria? are irregular in size and number, but there is a 

 general increase from the beaks to the base. Those near the base, indeed, almost merit 

 the name of costa3. The cardinal tooth is extremely small. 



A. LJEVIS, PL. 34. FIG. 14. 



A. testa transversd, sul-quadrilaterali, postice rolundatd, antice subtruncatd, convexa, inflata, l&vi; margine basali 

 recto; margine dorsali media angulato, postice et antice curvato; natibus sub-prominulis, incurvis; dente cardinali 

 longo; sulco lato, curto, pene transverso; margine interno integro. 



Shell transverse, sub-quadrilateral, posteriorly rounded, anteriorly sub-truncate, convex, inflated, smooth; basal 

 margin straight, dorsal margin angular in the middle, curved anteriorly and posteriorly; beaks somewhat prominent, 

 incurved; cardinal tooth long; sulcation wide, short, nearly transverse; margin entire within. 



Diam. .14. Length .18. Breadth .22 of an inch. 



Remarks. There are a few, almost imperceptible lines of growth over the surface of 

 the shell, which, under a powerful microscope, give it a somewhat roughened appearance. 

 The cardinal tooth is very long, and the sulcus is shallow, so that the hinge, at first 

 glance, is not unlike that of the genus Thracia. 



