390 GTL1V1AN A. DREW. 



pleted tlie first, step in the process of casting?. Tlie test cells, apical plate, and 

 stomodseum still adhere to tiie anterior end of the embryo. X 150. 



Fig. 35. — Lateral view of a reconstruction of an embryo that has completed 

 the process of castinj^. x 150. 



Fig. 30. — Lateral view of a reconstruction of an embryo in which the liver 

 pouches have begun to go to pieces. X 150. 



Fig. 37. — Sagittal section of an embryo in the stage represented by Fig. 36. 

 X 275. 



Fig. 38. — Transverse section of an embryo in the stage represented by Fig. 

 36, taken through the stomach just posterior to the pedal ganglia, x 275. 



Fig. 39. — Lateial view of a reconstruction of an embryo that is just begin- 

 ning to form the gills. X 150. 



Fig. 40. — Lateral view of a reconstruction of an embryo in which each gill 

 is beginning to form two lobes, x 150. 



Fig. 41.— Lateral view of a reconstruction of an embryo in which each gill 

 is beginning to form its third lobe. X 150. 



Fig. 42. — Horizontal section of an embryo in a stage represented by Fig. 



40, taken through the dorsal end of the stomach and the re-forming lobes of 

 the liver. X 200. 



Fig. 43. — Horizontal section of an embryo in the stage represented by Fig. 



41, taken ihrough the dorsal end of the stomach and the re-forming lobes of 

 the liver, X 200. 



Fig. 44. — Horizontal section of an embryo in the stage represented by Fig. 

 46, PI. 24, taken through the dorsal end of the stomach and the re-formed 

 lobes of the liver. X 150. 



PLATE 24. 



Fig. 45. — Lateral view of a reconstruction of an embryo in which each gill 

 has four pairs of plates, x 125. 



Fig. 46. — Lateral view of a reconstruction of an embryo in which each gill 

 has six pairs of plaies. X 125. 



Fig. 47. — -Lateral view of a reconstruction of an embryo in which each gill 

 has eight, pairs of plates. X 110. 



Fig. 48. — Lateral view of a reconstruction of an adult specimen, x 30. 



Fig. 49. — Adult specimen with the foot i)rotruded and tiie side flaps spread 

 apart, x 10. 



Fig. 50. — View of the inside of au adult left shell-valve, x 15. 



Fig. 51. — Left shell-valve seen obliquely from the dorsal margin, x 15. 



Fig. 52. — A nearly horizontal section of an embryo in the stage represented 

 by Fig. 40, cut to show the developing outer plates of the gills, x 150. 



