130 



CEETACEOUS LAMELLIBRANCHIA. 



from the umbo, thus increasing the total number of ribs at the margin of the 

 valve ; or (3) only a few smaller ribs occur here and there between the larger. The 

 first is associated with undivided ribs on the right valve ; the second with all the 

 ribs divided ; the third with a few ribs divided — the divided ribs of the right valve 

 corresponding in position with the small intercalated rib of the left valve. Rarely 

 one or two ribs on the left valve may be divided. In old specimens rather strong 

 growth-lines placed near together occur near the margin of the valve, giving it a 

 frilled appearance. Near the umbones the ribs sometimes have a serrated appear- 

 ance, due to the presence of short, close-set, spiny processes. Ears on both valves 

 with growth-lines only or with faintly marked radial ribs also near the umbones ; 

 at the inner border the ear is limited by a ridge (often indistinct on the left valve), 

 and between this and the commencement of the regular series of radial ribs on the 

 valve is a smooth area. 



Measurements of the left valve. 



Length (in millimetres) 



Height 



Number of main ribs 



Zone of T. gracilis. 

 49 48 47 47 35 34 

 53 50 51 49 38 39 

 83 43 27 30 33 27 



Zone of H. planus. 

 57 53 49 47 46 45 44 42 42 41 40 38 33 30 

 59 59 57 52 49 48 46 48 49 44 43 39 34 31 

 34 33 33 29 33 28 31 34 28 30 33 30 31 32 



Number of small intermediate ribs 9 8 8 11 5 8 ... 14 10 9 22 36 14 18 9 10 8 6 6 1 



Chali Eock. Zone of M. cor-testiidinarium. Zone of M. cor-anguinum. 



Length . . . . 51 46 37 



Height . . . . 62 50 41 



Number of main ribs . . 30 33 26 



Number of small intermediate ribs 11 11 8 



52 41 41 38 35 34 

 55 47 45 43 38 36 

 30 26 25 25 25 28 

 10 14 14 10 12 2 



72 62 55 51 51 50 47 



83 69 57 56 55 60 53 



30 35 34 26 31 27 29 



4 17 15 8 14 7 



Length . . . . 



Height .... 

 Number of main ribs 

 Number of small intermediate ribs 



Zone of Marsvjiites testiulinarius. 



53 53 53 47 47 47 45 44 43 43 43 42 40 37 35 



60 59 56 50 47 52 51 49 54 49 46 47 44 40 41 



30 29 31 29 29 37 28 28 29 28 26 26 27 27 35 



46 19 00 12 000 1 1 3 13 



Length . . . . 



Height . . . . 



Number of main ribs 

 Number of small intermediate ribs 



Zone of A. quadratus. 



55 40 33 29 27 26 



65 42 36 31 30 28 



35 28 27 27 31 29 



13 16 4 2 11 



Zone of B. mucronata. 



Tr: 



Iniingl 



lam Chalk. 



52 





36 



31 



56 





40 



32 



32 





30 



27 









4 







Remarks. — The amount of convexity of the two valves together, and also the 

 relative convexities of right and left valves, varies considerably in different 

 examples, and is to some extent associated with differences in the ribbing. On 

 the right valve the variation in the ribs consists (1) in the presence or absence 

 of a narrow furrow on the rib — giving the appearance of a split rib, — and (2) in 

 the period at which this furrow is introduced. Coincident with the beginning of 



