130 CRETACEOUS LAMELLIBRANCHIA. 
from the umbo, thus increasing the total number of ribs at the margin of the 
valve; or (3) only afew 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. 
Zone of T. gracilis. Zone of H,. planus. 
Length (in millimetres) . . 49 48 47 47 35 34 ... 57 53 49 47 46 45 44 42 42 41 40 38 33 30 
Height __,, 6 : . 53 50 5149 38 39 ... 59 59 57 52 49 48 46 48 49 44 43 39 34 31 
Number of main ribs ‘ . 83 43 27 80 38 27 ... 34 33 338 29 33 28 81 34 28 30 33 30 31 82 
Number of small intermediate ribs 9 8 811 5 8...1410 922361418 910 8 6 06 1 
Chalk Rock. Zone of M. cor-testudinarium. Zone of M. cor-anguinum. 
Length : j : > SI 46037 Be 52041 4138355840... (2062 Sb sols ol o0R47 
Height : 9 : . 6250 41 ... 55 47 45 43 388 36 ... 83 69 57 56 55 60 53 
Number of main ribs 5 » 80) 38726 4 80126 25.25) 25028). SOkooN 4a Zone adecD 
Number of small intermediate ribs 1111 8 ... 1014141012 2... 41715 0 814 7 
Zone of Marsupites testudinarius. 
Length : j 5 . 53 58 58 47 47 47 45 44 48 48 438 42 40 37 385 
Height ; : F . 60 59 56 50 47 52 51 49 54 49 46 47 44 40 41 
Number of main ribs : . 30: 29 31 29 29 37 28 28 29 28 26 26° 27 27 35 
Number of small intermediate ribs 4 619 0 012 0 0 0 0 1 1 0 8 18 
Zone of A. quadratus. Zone of B.mucronata, Trimingham Chalk. 
Length : é é . 55 40 33 29 27 26 005 52 ass 36 31 
Height : 6 : . 65 42 36 31 30 28 ae 56 sas 40 32 
Number of main ribs ; . 85 28 27 27 31 29 000 32 a8 30 27 
Number of small intermediate ribs 1316 0 4 2 11 aan 0 sis 4 0 
Remarks.—The amount of convexity of the two valves together, and also the 
relative conyexities of right and left valves, varies considerably in different 
examples, and is to some extent associated with differences in the ribbmg. On 
the right valve the variation in the ribs consists (1) im 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 
