part 4] CAMBEIAtf HORIZONS OF COMLET. 383 



Fig. 3. Ventral valve, internal cast [1708], with posterior view ; some shell 

 remains on the cast. 



4. Ventral valve, internal cast [1709], with lateral view ; much shell 



remains on the cast. 



5. Dorsal valve, internal cast, with lateral and posterior views [1707]. 



6. Ventral valve, interior [1711]. 



7. Do. do. do. [1706], the visceral cavity more rotund than 



usual. 



8. Ventral valve, internal cast, young individual, with lateral and 



posterior views. The visceral cavity is somewhat rotund [1716]> 



Acrothyra cf. sera (Matthew) Walcott, X 6, see p. 350 ; horizon Ac. 

 Fig. 9 a. Ventral valve, exfoliated slightly, with lateral and posterior views 



9 b. The same valve, from which all the shell has been removed, with 

 lateral view. 



10. Ventral valve, interior [2218]. 



11. Do. do. lateral view, posterior slope inclined backwards 



[1800]. 



12. Ventral valve, lateral view, posterior slope vertical [1799]. 



13. Do. do. exterior of apical portion [2207]. 



14 a. Dorsal valve, strongly exfoliated, with lateral outline [2207]. 

 14 6. The counterpart, showing many lamina? [2208]. 



15. Dorsal valve, internal cast, with lateral and posterior views [1801]. 



16. Smaller dorsal valve, partial exterior, with lateral view [1801]. 



17. Dorsal valve, internal cast, with lateral and posterior views, showing 



a greater extension of the median septum [1781]. 



18. Dorsal valve, exterior, with lateral view, much wider than usual and 



showing strongly-marked sculpture [18]. 



[The specimens represented by figs. 17 & 18 are referred 

 doubtfully to the species.] 



Acrothele coriacea Linnarsson, X 6, p. 345 ; horizons Bb 5 , Be, & Ca. 



Fig. 19. Ventral valve, partly exfoliated. The surface of the shell is seen 

 near the posterior margin where the sculpture is linear, towards 

 the apex it is granular ; a, summit; b, posterior view ; c, lateral 

 view [2345]. 

 19 d. Exfoliated interior of the same shell [2346]. 



20. Ventral (?) valve, fragment of external surface [2349], 



21. Dorsal valve, exfoliated, but showing two portions of the exterior, 



with lateral outline [2352]. 

 22 a. Dorsal valve, exfoliated, showing one of the two apical tubercles 

 and a slight median depression [2379]. 



22 b. The same specimen, after much shell- substance had been removed. 



Billingsella sp. indet.. X 3, see p. 354 ; horizon Bb x . 



Fig. 23. Ventral valve, exterior from a squeeze of the cast ; «, summit view; 

 b, lateral outline [1880]. 



23 c. Internal cast of the same [1879]. 



Billingsella lindstrcemi, var. salopiensis Matley, X 3, see p. 353 ; 

 horizons Bb 5 & Be. 



Fig. 24. Ventral valve, exterior from a squeeze of the cast ; a, summit view ; 

 /), lateral outline [2450], Be. 



24 c. Internal cast of the same [2449], Be. 



