PLATE IV 



RHYNCHONELLA WHITII, Hall 



Page 39 



Fig. i. The earliest observed stage of growth, the shell measuring 2.75 mm. in length by 2 mm in width. The deltidial plates 

 have already begun to form along the edges of the triangular foramen, and four pairs of plications are visible on the 

 dorsal valve. 



Fig. irt. Outline profile of the same. 



Fig. 2 Dorsal view of a normal adult having two plications on the fold; natural size. 



Fig. M. Profile of the same. 



Fig. 2/>. Front view of the same. (28th Kept. N. Y. State Mus. Nat. Hist., pi. 26, figs. 23, 26, 25.) 



Fig. 4. The cardinal features of a young example with a length of 3.25 mm.; showing the inception of the deltidial plates. 



Fig. 4. Profile of the same. 



Fig. 5. Cardinal features of an individual which has attained a length of 6 mm. The unibo has become incurved and the devel- 

 opment of the plates is well advanced, but it is arrested at this stage, the foramen not becoming inclosed at maturity 



Fig. 5<i. Outline profile of the same. 



RHYNCHONELLA NEGLECTA, Hall 



Page 37 



Fig. 6. Dorsal view of the youngest shell observed, having a length of .75 mm., a width of .5 mm. Though the foramen is open 

 and without evidence of thickened margins, four fine plications have already appeared on the dorsal valve. 



Fig. 6a. Outline profile of the same. 



Fig. 7, Dorsal view of an individual with a length of 2.25 mm. The deltidial plates are in an incipient condition, and the plica- 

 tions of the surface have considerably increased. 



Fig. 7<z. Outline profile of the same. 



Fig. 8. Dorsal view of a mature example; natural size. 



Fig. 8. The same in profile. (28th Rept. N. Y. State Mus. Nat. Hist., pi. 26, figs. 3, 5.) 



Fig. 3. A somewhat abnormal adult, having three plications on the fold and the lateral plications obsolescent, * 2 . (nth Ann. 

 Rept. Geol. Sur. Ind , pi. 27, fig. 3.) 



RHYNCHONELLA ACINUS, Hall 



Page 35 



Fig. 9. Ventral view of a young shell in a secondary stage of growth; showing a single growth-line and the embryonal median 

 ridge. 



Fig. ga. Dorsal view of the same. The foramen is slightly narrowed, but without evidence of deltidial plates; the embryonal 

 sinus is well defined. 



Fig. 96. Outline profile of the same. 



Fig. 10. Ventral view of a specimen 3.25 mm. in length. 



Fig. io(j. Dorsal view of the same. The mature fold and sinus have not yet begun to develop; the foramen shows increased con- 

 striction at its base, and slightly thickened margins. 



Fig. io. Outline profile of the same. 



Fig. ii. Dorsal view of an average adult, X4. 



Fig. im. Profile of the same. 



Fig. 116. Front view of the same; showing the elevation of the median fold, 



RHYNCHOTRETA CUNEATA (Dalman), var. AMERICANA, Hall 



Page 47 

 See Plate viii 



Fig. 12. Dorsal view of a shell representing the earliest stage of growth observed (length i . 5 mm.). The forann-n is widely tri- 

 angular and unobstructed; the surface is covered with all the plications of maturity, and the subcircular valve bears 

 a broad median depression. 



Fjg. iza. Outline profile of the same. 



Fig. 13. Similar view of a shell 3 mm. in length. The outline has become elongate, the foramen narrowed and its margins thickened. 



Fig. 13*1. Outline profile ot the same. 



Fig. 14. Dorsal view of a normal adult individual, natural size; showing the elevated beak and broad plications. 



Fig. i4. Profile of the same, showing the elevation of the dorsal fold. 



Fig. 14^. Front view of the same (28thRept.N. Y. State Mus. Nat. Hist., pi. 25, figs. 31, 34, 36.) 



Fig. 15. A series of outlines of the anterior margin ; showing the embryonal sinus or depression in the dorsal valve in young 

 forms, reaching its maximum in individuals having a length of 4.5 and becoming obsolete, or the line of junction 

 straight, in specimens 7 mm. in length. Outline 6 snows the inception of the dorsal fold. In 8 the four median pli- 

 cations are distinctly elevated ; 9 is from a normal full-grown individual and 10 represents the maximum elevation of 

 the fold. The outlines, except in 9 and 10, are enlarged to the diameter of a fully developed specimen. 



Fig. 16. Cardinal area of fie. 12. 



Fig. i6rt. Outline profile of the same. 



Fig. 17. Cardinal area of fig. 13. 



Fig. i-ja. Outline profile of the same. 



Fig. 18. Cardinal area in an individual 4.5 mm. in length; showing the considerably advanced development of the deltidial plates 

 and the encroachment of the foramen on the apex of the valve. 



Fig. i8rt. Outline profile of the same 



Fig. 19. Cardinal area at a length of 5 mm. 



Fig. iga. Outline profile of the same. 



Fig. 20, Cardinal area at a length of 7 mm. 



Fjg. 20rt., Outline profile of the same. 



Fig. 21. Cardinal area in an individual- which has attained a length of 12 mm. The plates are united for nearly one-half their 

 length, and slightly bent outward about the base of the foramen. 



Fig. 2i(z. Outline profile of the same. 



Fig. 22. The cardinal area at maturity; showing a stronger flexion of the plates below the foramen, and a subcircular foramen, en- 

 croaching upon the apex of the shell. 



Fig. 22d. Outline profile of the same. 



