470 



THE GEOLOGIST. 



7. Cyrtia trapezoidalis, Dalman. Interior of the ventral valve, to show that the 



diverging dental or rostral plates are similar to those of Sinrifera, 

 but essentially different from those of Cyrtina. D, pseudo-del- 

 tidium ; F, foramen. 



8. „ cuspidata, Martin, viewed in profile, and from which a portion of the 

 *' valves have been removed, so as to exhibit the two diverging 



dental or rostral plates. 



9. SpiB.iii'SRiNA rostrata, Schlotheim, from which a portion of the smaller valve has 



been removed, so as to show the back of the spiral cones, as well 

 as the position of the central plate of the larger or ventral valve. 

 ,^ rostrata. Interior of the ventral valve, exhibiting the large ver- 

 tical median plate, or septum, to which was attached the adductor 

 or occlusor muscle. R, ventral adjus'cor. The two plates com- 

 posing the pseudo-deltidium are here clearly defined. 



11. SuBSSiA j!?Zicato, E. Deslongchamps. Interior of the dorsal valve, in which 



the cardinal process and largely-developed hinge-plates are well 

 exhibited ; no perfect interior of this genus has, however, been 

 obtained, so that the spiral appendages could not be represented. 

 A good illustration of the larger valve is likewise among the 

 desiderata, although some of its characters have been described 

 and represented by Mr. E. Deslongchamps. 



12. Cyrtina lieteroclyta, Defrance. Interior of the ventral valve, to show the large 



single median septum, to the upper surface of which were 

 attached dental or rostral plates ; a small portion of the arched 

 pseudo-deltidium has been removed. 



13. „ Demarlii, Bouchard, to show that in this species tlie median septum 



extends to the roof of the arched pseudo-deltidium, the dental 

 plates being fixed to the sides of the septum, instead of the upper 

 edge, as in C. heteroclyta and C. septosa. The perfect interior 

 of the smaller or dorsal valve of no species of Cyrtina has been 

 hitherto discovered. 



14. Athyris (M'Coy) = SpiRiGERA {di'Oyih) pectinif era, J. de C. Sowerby. Interior 



of the dorsal valve, and of a portion of the ventral one, magnified. 

 The hinge-plate, and its small circular aperture, as well as the 

 spirals and their complicated connecting processes, are here repre- 

 sented ; but the spiral cone on the right side has been somewhat 

 displaced from its natural position in order to better explain the 

 connecting system of lamellse. The hinge of both valves is also 

 here seen, as well as the rostral plates and foramen of the larger 

 or ventral valve. 



15. „ concentrica, Von Buch. Interior of the ventral valve ; the adductor 



A, and divaricator muscular impressions R, are here clearly 

 defined. 



16. Merista Uerculea, Barrande, to show the shoe-lifter-shaped process H, 



first described by Prof. King. 



17. tumida, Dalman. Interior of the dorsal valve. Although much 



labour has been bestowed by Prof Suess in working out the 

 interior character of his genus, a correct restored representation 

 of the interior has still to be produced ; the one here given is not 

 sufficiently detailed or precise. 



18. „ Interior of the ventral valve. 



19. „ scalprum, Rcemer. Exterior of the ventral valve, in which the two 



diverging lines due to the dental plates are seen, as well as the 

 hollow under the shoe-lifter process or plate. 



20. Retzia serjyejitina, de Koninck, from a drawing by Mr, S. P. Woodward ; no 



perfect representation of the interior of either valve has been 

 hitherto produced, and all we know is, that the specimens com- 

 posing the sub-genus were provided with spiral processes. 



