132 CRETACEOUS LAMELLIBRANCHIA. 
N. spinosus, having the valves rather more convex than usual and the ribs on the 
ears a little more distinct. 
Two small specimens in Dr. Blackmore’s collection from the A. quadratus zone 
of Hast Harnham (length 6 mm.), and the B. mucronata zone of Clarendon (length 
65 mm.), are probably young individuals of S. spinosus. 
S. superbus, Willett, MS., is an example of the equalis-type from the Upper 
Chalk (probably Marsupites zone) of Seaford. 
The inner layer of the shell, and also the area and teeth, have been removed by 
solution from the specimens of S. spinosus and other species of Spondylus found in 
the Chalk of England.' 
A large number of good specimens of S. spinosus are preserved in most 
museums and collections, but unfortunately, in the majority of cases, the zones 
from which they were obtained are not definitely known, and consequently, in 
order to determine whether any of the varieties are characteristic of particular 
zones, I have had to rely mainly on the collections kindly lent me by Drs. Black- 
more and Rowe, and Messrs. Jukes-Browne, W. Hill, G. HE. Dibley, and J. Scanes, 
and on my own collecting. Similarly, in selectmg specimens for figuring, I have 
chosen those of which the exact horizon is known even when finer examples of the 
same type, but of uncertain zone, were at hand. In order to work out satisfactorily 
the zonal characters of S. spinosus, a much larger number of carefully collected 
specimens than I have had at my disposal would be required, and the following 
notes on the forms found in successive zones must, therefore, be regarded as of a 
preliminary nature only. 
T. gracilis-zone-—The common forms (Pl. XXIII, fig. 6) have the valves 
flattened. Ribs on the left valve are generally slender and separated by broad 
erooves; intermediate (small) ribs are seen in all cases, and, as a rule, are 
moderately numerous. Forms with the left valve more convex occur rather rarely 
(PIS XT fies 7): 
H. planus-zone and Chalk Rock.—Valves, especially the left, are more convex 
than in the preceding zone; ribs rather stouter (Pl. XXIII, figs. 8—10). Some 
forms having few or no intermediate ribs occur (Pl. XXIII, fig. 8). 
M. cor-testudinarium-zone.-—Commonly the left valve is very convex, and the 
intermediate ribs are introduced early and become nearly as large as the others 
(Pl. XXITI, fig. 11). Less common are forms with only a few intermediate 
ribs. 
M. cor-anguinum-zone.—Convexity of the two valves is generally more nearly 
equal. Intermediate ribs are generally small and not numerous, and the main ribs 
stout (Pl. XXIV, fies. 1, 2). The old individuals of this type, named Plagiostoma 
1 For a more detailed account of this, with references to previous writers on the subject, see 
Woods, ‘ Quart. Journ. Geol. Soe.,’ vol. lui (1897), p. 386. 
