PLATE XVI 

 LEPT^ENA RHOMBOIDALIS WILCKENS (PAGES 322-327) 



FIGURE 1. Ventral view of the youngest shell observed, its length 

 being 1.25 mm. The aperture of the embryonal pedicle-sheath is very 

 conspicuous, and its margins are very thick. The surface shows a faint 

 median depression, indications of two concentric growth-lines, and out- 

 side the latter of these, obscure traces of plications. 



FIGURE la. Outline profile of the same; showing the prominence of 

 the sheath. 



FIGURE 2. Ventral view of an individual with a length of 2 mm. 

 The aperture of the pedicle-sheath is relatively somewhat diminished in 

 size, its margins have become thinner, and the radiating plications nu- 

 merous and sharply defined. 



FIGURE 2a. Outline profile of the same; indicating diminution in 

 the prominence of the sheath. 



FIGURE 3.. Ventral view of an individual having a length of 4 mm. ; 

 showing the increase in the number of plications, the appearance of nu- 

 merous concentric undulations and strise, and the narrowing pedicle- 

 aperture. 



FIGURE 3a. Outline profile of the same ; showing the concentric 

 undulations and the diminishing pedicle-sheath. 



FIGURE 4. Ventral view of a normal adult, having a length of 

 28 mm. ; showing the characters of maturity. Natural size. 



FIGURE 4. Profile of another individual of full growth ; showing 

 the anterior geniculation and the length of the anterior slope, or curtain. 

 (28th Kept. N. Y. State Mus. Nat. Hist., pi. 22, figs. 6, 7.) 



FIGURE 5. Cardinal view of the specimen represented by figure 1 . 



FIGURE 6. Similar view of the specimen represented by figure 2. 



FIGURE 7. Similar view of an individual 2.65 mm. in length. 



FIGURE 8. Similar view of the specimen represented by figure 3. 



FIGURE 9. Similar view of an individual 9 mm. in length. 

 These cardinal views are drawn with the same degree of enlargement, 

 and show the gradual diminution in height and in diameter of aperture 

 in the pedicle-sheath, and the increasing development of the grooved 

 callosity on the dorsal valve. 



FIGURE 10. Cardinal view of a normal adult; showing the great 

 size of the grooved callosity, and the csecal opening, representing the 

 atrophied pedicle-sheath. Natural size. (op. cit., fig. 10.) 



