NO. 2094. MOLLUSKS OF THE GENUS RIS80INA—BART8CH. 37 



i^. Intercostal spaces double the -width of the ribs. 



/. Basal cords 11 gisna, p. 41. 



j^. Basal cords more than 15 inca, p. 42. 



K'. Basal sculpture not uniform, consisting of sti'ong 



spiral cords and fine lines between them favilla, p. 43. 



g^. Shell not broadly conic but elongate conic. 

 h^. Spiral sculpture of the spii'e exceedingly fine. . .mazatlanica, p. 43, 



h'^. Spiral sculpture of the spire strong helena, p. 44. 



/^. Base not evenly curved, with a decidedly tumid area 

 or keel anteriorly. 

 g^. Base with a tumid area on the anterior third. 



h^. Adult shell 9 mm. long lo, p.44. 



¥. Adult shell 7 mm. long dina, p. 45. 



g'^. Base with a strong keel at the insertion of the colu- 

 mella signae, p. 61. 



c^. Summit of the whorl not strongly shouldered but ap- 



/^ Shell elongate-ovate expansa, p. 46. 



/^. Shell not elongate-ovate. 

 g^. Shell elongate-conic. 

 h^. Axial ribs less than 32. 



i^. Last whorl inflated peninsular is, p. 46. 



i^. Last whorl not inflated adamsi, p. 47. 



h''. Axial ribs more than 42 townsendii, p. 48. 



gr^. Shell not elongate-conic but broadly conic barthelowi, p. 48. 



(P. Axial sculptm-e not consisting of prominent ribs; ribs obso- 

 lete. 



d^. Whorls shouldered kelseyi, p. 49. 



d^. Whorls not shouldered but overhanging lapazana, p. 50. 



6^. Spiral sculpture on base only. 

 c^. Axial ribs present. 



d^. Base with two keels contabulata, p. 51. 



d^. Base with one keel and two incised spiral lines histia, p. 51. 



d^. Base with many spiral cords zeltneri, p. 52. 



c^. Axial ribs absent laurae, p. 52. 



a-. Spiral sculpture absent. 



¥. Axial sculpture consisting of strong ribs, 

 c^ Axial sculpture consisting of strong ribs only. 

 d^. Summit of the whorls shouldered. 



e\ Axial ribs lamellar hurragei, p. 52. 



e^. Axial ribs not lamellar nereina, p. 53. 



d'. Summit of the whorls not shouldered. 



e^. Whorls decidedly constricted immediately anterior to 



the summit effusa, p. 54. 



e^. Whorls not or scarcely at all constricted anterior to the 

 summit. 



/^ Adult shell more than 4.5 mm. long pleistocena, p. 54. 



/^. Adult shell 3 mm. or less. 



g^. Anterior portion of base bordered by a fasciole cali/omica, p. 55. 



g"^. Anterior portion of base without fasciole. 



/i^ Axial ribs on last whorl 20 dandestina, p. 55. 



h'^. Axial ribs on last whorl 16 mexicana, p. 56. 



