380 MR. E. S. COBBOUD ON THE [vol. lxxvi, 



EXPLANATION OF PLATES XXI-XXIV. 



[The numerals in brackets are those attached to the specimens. Those with 

 the letters PR before them refer to specimens collected by Mr. Rhodes for 

 the Geological Survey. The name ' King ' before a number indicates that 

 the specimens had been collected by Mr. Wickham King, and presented by him 

 to Mr. Rhodes's collection. A few specimens belong to the British Museum, 

 and all others are from the collection for the British Association, which 

 is in process of being deposited in the Sedgwick Museum, Cambridge.] 



Plate XXI. 

 Micromitra (Paterina) phillijisi HolL sp., X 4, see p. 329 ; horizon Ab r 



Pig. 1. Ventral valve, with lateral view [PR 1472]. 



2. Do. do. with lateral and posterior views [PR 1467]. 



3. Do. do. internal cast, with lateral view [2334]. 



Micromitra (Paterina) rhodesi, sp. nov., X 4. see p. 330 ; horizon Abj. 



Pig. 4. Type-specimen, ventral valve ; a, summit view ; b, posterior view ; 

 c, lateral view [PR 1473]. 



5. Ventral valve, alate form, from a squeeze of an external cast in soft 



sandstone [2271]. 



6. Ventral valve with lateral view, shell somewhat flattened and im- 



pressed with sand-grains, apex broken, revealing the thickness of 

 shell [PR 1471]. ' 



Micromitra (Paterina) hingi, sp. nov., X 4, see p. 330 ; horizon Ab r 



Pig. 7. Type-specimen, ventral valve with lateral view [King 2]. 



8. Ventral -\ alve ; a, summit view ; b, posterior view, showing pseudo- 

 deltidium [King 2]. 



Micromitra sp., X 4, see p. 330 ; horizon Abj. 



Pig. 9. Dorsal valve, with lateral and posterior views, associated with the 

 previous specimens, shell exfoliated [RR 1465]. 



Micromitra (Paterina) labrddorica Billings, X 4, see p. 332 ; 

 horizons Ac x to Ac-. 



Fig. 10. The two valves in contact : a, ventral ; b, lateral ; c, dorsal views 

 [British Museum Collection, B. 40306], Ac.,. 



11. Dorsal valve, exfoliated, and showing radiate stria? on five successive 



layers of shell-material [2241], Ac 2 . 



12. Part of surface of valve in which the rounded concentric bands vary 



much in width, enlarged to 8 diameters [730], Ac,,. 



13. Ventral valve, with lateral and posterior views. The false area 



is complete, and shows a minute shelf -like pseudodeltidium pro- 

 jecting horizontally under the umbo [2243], Ac.,. 



Micromitra (Paterina) minor, sp. nov., X 8, see p. 333; horizon Ac,. 



Fig. 14. Type-specimen, ventral valve, exterior, with lateral and jjosterior 

 views [755]. 



15. Surface of another specimen [RR 1242], X 16. 



16. Ventral valve exfoliated at the apex; a, summit view; b, lateral 



view; c, the exfoliated apex X 16 showing casts of vascular 

 marks and median depression [750]. 



Micromitra (Paterina) minor, var. gibbosa nov., X 8, see p. 334 ; horizon Ac.,. 

 Fig. 17. Ventral valve, summit and lateral views [RR 1117]. 



