PLATE I 

 Crania siluriana, Hall 



P«Re 13 

 Ki|{. I. The younfi^est indi\-idual observed; havioK a height of 1 mm. and a width across the base of 1.5 mm. The elevation of 



the shell is m strong contrast to that of the mature form. 

 Fi(f. 3. A mature individual attached to a shell ol' Platystoma (28th Kept. N. Y. State Mus. Nat. Hist., pi. 21, fig. 5). 



Orthis elegantula, Dalman 



Page J3 

 See Plate viii 



Fig. 3. Dorsal view of the youngest embryo observed, its length being .5 mm. ; its width .75 mm. The median sinus has already 

 formed, and three pairs of plications have appeared, of which the middle pair is evidently the oldest. 



Fig. 3a. Outline profile of the same; showing the slightly greater convexity of the ventral valve. 



Fig. 4. A larger example viewed from the dorsal side, its length being i mm., its width 1.5 mm. The plications have now in- 

 creased to the number of six pairs, one of which has appeared between the median plications seen in fig. 3. 



Fig. 5. Ventral view of an individual in about the same stage of development; showing a strong median plication corresponding 

 to the dorsal sinus, and five pairs of lateral plications. 



Fig. 6. Cardinal view of an individual in the growth-stage represented by fig. 5. The valves have nearly the same convexity, 

 while the width of the cardinal area and the size of the pedicle-passage are the same for each. The latter is seen to be 

 quite unobstructed and without further differentiation that a slight thickening of the margins. 



Fig. 6a. Outline profile of the same. 



Fig. 7. Cardinal view of an individual which has reached a size of 3 x 3 mm. Here is apparent a change in the development of 

 the cardinal area and foramen of the valves. The primary indication of the callosity or cardinal process is in the 

 apex of the dorsal opening. The differenc-e in the convexity of the valves has also noticeably increased. 



Fig. fa. Outline profile of the same. 



Fig. 8. Cardinal view at a size of 5 x 5 mm. The ventral beak has become strongly incurved, and the cardinal process is now 

 subdivided into three parts. The cross-lines, representing the natural size of the specimen, are too short. 



Fig. 8a. Outline profile of the same. 



Fig. 9. Cardinal view at a size of 12 x n mm. The ventral valve and area have become greatly curved, and the dorsal aperture 

 is quite filled by the tripartite cardinal process. 



Fig. 9a. Outline profile of the same. 



Fig. «o. Cardinal view of a large adult, size 18x18. 5mm. The areas are closely appressed, and the dorsal aperture is wholly filled 

 by the cardinal process, the central portion of which extends into the aperture of the other valve. 



Fig. loa. Outline profile of the same. 



Fig. II. Cardinal view of a normal adult; lutural size. 



Fig. iia. Profile of the same {op. cit. pi. 21, figs. 17, 14). 



Fig. 12. Dorsal view of a small adult. 



Fig. 12a. Ventral view of the same {of. cit., figs. 12, 11). 



Orthis hybrida, Sowerby 



Page 17 



Fig. 13. Cardinal view of a very young individual having a length of i mm. and a width of 1.5 mm. The valves are nearly 

 equiconvex, the area and apertures as in the earlier stages of O. elegantula. 



Fig. 13a. Outline profile of the same. 



Fig. 14. Cardinal view of a somewhat gibbous example measuring 10 x 8 mm. The relatively short areas are about equally de- 

 veloped, and the cardinal callosity of the dorsal valve has already filled the dorsal aperture. 



Fig. 14a. Outline profile of the same. 



Fig. IS. Cardinal view of a normal adult 12 x 10 mm.; showing the short area and the projection of the cardinal process into the 

 ventral aperture. 



Fig. ISO. Outline profile, showing the incurvature of the areas. 



Fig. 16. Dorsal view of a large adult {op. cit. fig. 20}. 



Fig. 17. Profile of a normal adult (op. cit. fig. 21). 



Fig. 18. Dorsal view of a small, gibbous example, showing strong varices. Enlarged to two diameters. 



Fig. iga. Profile of the same. 



