382 MB. E. S. COBBOLD ON THE [vol. lxxvi, 



Fig. 15. Ventral valve, internal cast, with lateral outline [2200], Ac.,. 



16. Minute ventral valve, internal cast, young and rather wider propor- 



tionally than the adult [2198], Ac.,. 



17. Ventral valve, exterior, with lateral outline [760], Ac.,. 



18. Do. do. do. do. do. do. [1755], Ac'-. 



19. Dorsal valve, exterior, with lateral outline [1756], Ac,. 



Obolus (?) gibbosus, sp. nov., X 6, see p. 339 ; horizon Be. 



Fig. 20. Type- specimen, ventral valve, exterior, with lateral view [2119]. 

 21. Dorsal valve, internal cast, with lateral outline [2120]. 



Obolus (?) cf. schmalenseei Walcott, X 6, see p. 340 ; horizon Be. 

 Fig. 22. Dorsal valve, internal cast ; a, summit view ; o, lateral outline [689], 



Obolus (?) linnarssoni, sp. nov., X 4 and X 6, see p. 338 ; horizon Be. 



Fig. 23. Ventral (?) valve, X 4, variously exfoliated, with lateral outline, 

 showing at a, portion of exterior ; at bb, the two raised bands on 

 an inner layer [BE 1868]. 



24. Dorsal (?) valve, X 6, much exfoliated [2385]. 



25. Minute valve, X 6, doubtfully referred to the species [2399]. 



Acrotreta schmalenseei var. matleyi, nov., X 6, see p. 346 ; horizons 

 Be & (?) Bb 5 . 



Fig. 26. Type-specimen, ventral valve, exterior, with lateral and posterior 

 views [2066]. 



27. Ventral valve, exterior, exfoliated and apex removed, with lateral and 



posterior views [2076]. 



28. Ventral valve, internal cast, with lateral and posterior views [2109]. 



29. Dorsal valve, internal cast, with lateral outline [2079]. 



30. Do. do. exterior somewhat exfoliated, with lateral outline 



[2078]. 



31. Dorsal valve, exterior, wider and with surface more strongly marked, 



doubtfully referred to the species [2103]. 



32. Surface of a ventral valve, X 12 [2087]. 



Acrotreta sp. indet. a, X 6, p. 347 ; horizon Be. 

 Fig. 33. Ventral valve, internal cast, with lateral and posterior -views [2093]. 



Acrotreta sp. indet. c, X 6, see p. 34S ; horizon Bb 2 . 

 Fig. 34. Dorsal valve, internal cast [1621]. 



Acrotreta socialis Seebach, X 6, see p. 347 ; horizon Ba :i . 

 Fig. 35. Ventral valve, with lateral and posterior views [1640]. 



Plate XXIII. 



Acrotliyra comleyensis, sp. nov., X 6, see p. 348 ; horizons Bb, & Bb 3 . 



Fig. 1. Five drawings of the apical part of a ventral valve [1714], from 

 which successive layers of shell were removed ; a, exterior ; 

 b, first exfoliation, showing, so far as the curved line, the internal 

 cast of a young shell ; c, second exfoliation, showing internal cast 

 of a small shell attaining adult features ; d and e, further 

 exfoliations, showing further stages of growth. 

 2. Type-specimen, ventral valve, internal cast [1710], from which all 

 the shell has been removed, with lateral and posterior views. 



