PLATE II 



Strophomena rhomboidalis, Wilckens 



Pafce 18 



Fig. I. Veatral view of the youngesi shell observed, its length being 1.35 mm. The aperture of the embryonal pedicle-sheath 

 is very conspicuous, and its margins very thick. The surface shows a faint median depression, indications of two con- 

 centric growth-lines, and outside the latter of these, obscure traces of plications. 



Fig. irt. Outline protile of the same, showing the prominence of the sheath. 



Fig. 2. Ventral view of an individual with a length of 2 m m. The aperture of the pedicle-sheath is relatively somewhat dimin- 

 ished in size, its margins have become thinner, and the radiating plications numerous and sharply defined. 



Fig. 2*. Outline profile of the same, indicating diminution in tlie prominence of the sheath. 



^Ui- 3* Ventral view of an individual having a length of 4 mm.; showing the increase in the number of plications, the appearance 

 of numerous concentric untlulations and slriar, and the narrowing pedicle-aperture. 



Fig. jrt. Outline profile of the same, showing the concentric undulations and the dmiinishing pedicle-sheath. 



Fig. 4. Ventral view of a normal adult, natural size, having a length of 28 mm.; showing the characters of maturity. 



Fig. 4a. Profile of another individual of full-growth; showing the anterior geniculation and the length of the anterior slope or 

 curtain. (aSlh Rept. N. V. State Mus. Nat. Hist. pi. as, figs. 6, 7.) 



Fig. 5. Cardinal view of the specimen represented by fig. i. 



Fig. 6. Similar view of the specimen represented by fig, a. 



Fig. 7. Similar vie a- of an individual 2.65 mm. in length 



Fig. 8. Similar view of the specimen represented by fig. 3. 



Fig. 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 de- 

 velopment of the grooved callosity on ihe dorsal valve. 



Fig. 10. Cardinal view of a normal adult, natural size; showing the great size of the grooved callosity, and the ciecal opcnmg, 

 representing the atrophied pedicle-sheath {o/>. cit. fig. 10). 



Fig, II. The cardinal area roi>re8ented In fi^. 5 (length i- 35 mm.), atitl further eplarg«n; snowing 'tHe btoad.bromment, exscrt 

 sheath, embracing, at its base, the faint, grooved dorsal callosity. 



Fig. 12. The cardinal area shown in fig 6 (.length 2' mm.), enlarged to the size of fig. 11; showing the depression of the sheath, 

 the narrowing of the cardinal area, and the increasing aperture between the sheath and callosity. 



Fig. 13. The pedicle-area of a mature individual, X2- The sheath is now wholly absorbed, the sole trace of it being seen in the 

 caecal foramen, surrounded by the umbonal portion of the shell. The callosity is strongly developed, but not suffi- 

 ciently to close the gap between it and the opposite valve, thus leaving a passage between the valves and along the 

 dorsal groove. 



STREPTORHYNCHUS SUBPLANUM, Conrad 



Page 23 

 Sec Plate viii 



Fig. 14. V^entral view of the smallest individual observed, having a length of 3.25 mm. Both primary and secondary plications 

 and concentric growth-lines have already appeared, indicating the very early assumption of these characters 



F^. 144. Outline profile of the same; lowing the convexity of the valves. 



Fig. 15. A normal adult, dorsal view. 



Fig. 15*1. The same in profile Kop. cit. pi. 21, figs. 30, 31). 



Fig. 16. Cardinal view of specimen somewhat larger than that represented in fig. 14. The ventral valve bears a small pedicle- 

 sheath, the dorsal, the inception of a cardinal process or callosity, while between the two is a broad opening, which 

 serves to imlicate that, at this early age, the pedicle-sheath had ceased its function. 



Fig. 17. Cardinal view of an individual slightly below normal full growth, but with essentially mature characters. 



Fig. 18. The pedicle-area of the specimen represented in fig. 16. 



Fig. 19. Pedicle-area of a shell having a length of 4 mm. At this stage of growth the sheath has relatively diminished in size, 

 while the dorsal callosity has increased and shows a median groove on its inner edge. Deltidial plates have also be- 

 gun to develop along the margins of the ventral aperture. 



Fig. 30. Pedicte-area of the specimen represented in fig. 17. The sheath is now atrophied and altogether obsolete, the dorsal 

 callosit/ is very large, nearly filling the aperture between the valves, and the deltidial plates have attained the max- 

 imum development observed in the Strophomcnido;. 

 The last three figures have the same degree of enlargement. 



