PLATE IX. 



Fig. 



1 . Trigonotreta undulata, /. de C. Sowerhy. Testiferous specimen, natural size, show 



ing area (a), and fissure or deltidium (<5). Midderidge. 



(This beautiful specimen, which is the original of the figure in the 

 ' Mineral Conchology' (PL 562, fig. 1), has been kindly lent me by 

 its owner, Mr. J. de C. Sowerby, for illustration. I have carefully 

 removed some of the investing matrix, thereby showing certain parts 

 more distinctly than in the figure just cited.) 



2. -^ — Large valve of the same specimen. 



3. — — Lateral aspect of the same. 



4. Trigonotreta alata, Schlotheim. Cast, natural size, showing area, fissure, and small 



valve. Humbleton Hill. 



5. — — Cast (natural size) of the large valve. Humbleton Hill. 



6. — — Cast (natural size) of the small valve, showing impres'sions 



of the anterior division of the valvular muscles {a) ; 

 posterior division of the same {b) ; and vascular 

 system {c). Humbleton Hill. 



7. — — Gutta-percha cast of the last specimen, showing area (a) ; 



excavated (? cardinal) muscular impressions {b) ; and 

 origin of the crura of the spirals (c). 



8. — — Cast, natural size, showing one of the spiral coils. Hum- 



bleton Hill. 



9. — — Testiferous specimen, natural size, showing laminae of 



growth. Midderidge. 



10. — — Cast of a young specimen, rather enlarged, showing ribs 



parallel to the hinge-line. Humbleton Hill. 



11. — — Cast, natural size. Humbleton Hill. 



12. — — Gutta-percha cast of the medial region of the area, showing 



the fissure closed inwardly by an arch-shaped plate. 



(The lower cone-like portion bounded by the dark line, should not have 

 been represented as it is foreign matter). 



13. Trigonotreta undulata, /. de C. Sow. Specimen, natural size, with the shell partially 



removed. Humbleton Hill. 



14. — — Cast, natural size. Humbleton Hill. 



15. — — Testiferous specimen, natural size. Tunstall HUl. 



16. — — Young specimen, rather enlarged. Tunstall Hill. 



17. — — Portion of the shell, highly magnified, showing the laminae 



to consist of capillary fibres. Tynemouth Cliff. 



18. Trigonotreta Permiana, Xe;2y. Cast, natural size. Humbleton Hill. 



19. — — Cast (natural size) of large valve. Humbleton Hill. 



20. — — Cast, natural size. Humbleton Hill. 



21. — — Cast (natural size) of large valve. Humbleton Hill. 



22. — — Small valve of the last specimen. 



23. — — Cast, twice the natural size. Humbleton Hill. 



24. — — Large valve of last specimen. 



