132 CRETACEOUS LAMELLIBRANCHIA. 



;S\ spiiiosns, 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. quadvatus zone 

 of East Harnham (length 6 mm.), and the B. mucronata zone of Clarendon (length 

 0'5 mm.), are probably young individuals of S. spinosus. 



S. sujjerhus, Willett, MS., is an example of the sequalis-tjipe 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 8. spinomis and other species of Spoiidyhis found in 

 the Chalk of England.^ 



A large number of good specimens of 8. 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, Gr. E. Dibley, and J. Scanes, 

 and on my OAvn collecting. Similarly, in selecting 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 8. 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. (jracUis-zowQ. — The common forms (PI. XXIII, fig. G) have the valves 

 flattened. Ribs on the left valve are generally slender and separated by broad 

 grooves; 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 

 (PI. XXIII, fig. 7). 



H. plaiius-zone and Chalk Rock. — Valves, especially the left, are more convex 

 than in the preceding zone; ribs rather stouter (PI. XXIII, figs. 8 — 10). Some 

 forms having few or no intermediate ribs occur (PI. XXIII, fig. 8). 



M. cor-tesf/adinarium-zone. — Commonly the left valve is very convex, and the 

 intermediate ribs are introduced early and become nearly as large as the others 

 (PL XXIII, fig. 11). Less common are forms with only a few intermediate 

 ribs. 



M. cor-anguinimi-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, figs. 1, 2). The old individuals of this type, named Plagiostoma 



^ For a more detailed account of this, with references to previous writers on the subject, see 

 Woods, ' Quart. Journ. Geol. Soc.,' vol. liii (1897), p. 386. 



