PECTEN. 183 
much smaller, triangular, outer angle obtuse, marked with radial striz lke those 
on the valve. 
Left valve rather more convex, ornamented with similar but fewer ribs, bearing 
scaly spines which are often placed at fairly regular intervals. Interspaces with 
striz as on the right valve. Ribs generally of nearly uniform size; but im some 
sases smaller ribs are intercalated at some distance from the umbo, so that alternat- 
ing larger and smaller ribs are clearly seen. Narrow concentric ridges, placed at 
regular intervals, are sometimes present near the umbo. Anterior ear large, with a 
few radial ribs—usually near the valve only, leaving the upper part smooth; ribs 
crossed by oblique striz. Posterior ear small, with outer angle obtuse, and marked 
with striz. 
Remarks.—This species varies greatly ; the number of ribs on the left valve 
ranges from 35 to 57, and consequently the width of the interspaces also varies. 
Sometimes smaller ribs are intercalated between some of the others. The spines 
on the ribs may be close together and numerous, or fewer and more widely sepa- 
rated, and sometimes they have a fairly distinct concentric arrangement ; im some 
rases the ribs on part of a valve, or on almost the entire surface, are without 
spines. Such smooth or nearly smooth forms occur in the Hythe Beds of Hythe, 
the Chloritic Marl of Maiden Bradley, the Rye Hill Sand of Warminster, the 
Cenomanian (Bed 12) of Beer Head, ete. The sharpness of the spines varies in 
different specimens, and is partly dependent on the nature of the enclosing rock. 
Some examples from Faringdon and also some from the Upper Greensand 
(Pl. XXXV, fig. 3) have ribs over the whole of the anterior left ear, and the 
ribs on the valve are numerous with the spines irregularly placed; I have not 
sufficient specimens to determine whether these are distinct from the other forms, 
or merely a variety. Some forms (Pl. XXXYV, fig. 1) with few and strong ribs, 
and well-developed scales, also appear at first sight to be distinct, but it is difficult 
to draw a line between them and the normal form.’ A much larger series of 
specimens is required to determine whether these forms are really distinct, or 
merely varieties. 
Mr. Jukes-Browne now agrees with me in regarding the forms which he named 
' A form similar to this, but having more numerous ribs and with the spines usually more 
pointed and erect, obtained from the Cenomanian Sandstone of Wilmington, ete. (Pl. XXXV, 
fig. 10) has been referred by Jukes-Browne to P. hispidus, Goldfuss. The state of preservation of the 
specimens is not sufficiently good to enable me to decide as to their affinities, but Iam inclined to 
regard them as constituting only a variety of P. Robinaldinus. In some examples of the latter the 
spines are almost, if not quite, as large as in the former. hey agree better with the figures of 
Geinitz than of Goldfuss, but the ribbing appears to vary considerably. Goldfuss, * Petref. Germ.,’ 
vol. ii (1833), p. 59, pl. xciv, fig. 4; Pictet and Campiche, ‘Foss. Terr. Crét. Ste. Croix’ (1870), 
p. 215; Geinitz, ‘Das Elbthalgeb. in Sachsen’ (‘ Paleontographica,’ vol. xx, pt. 1, 1872), p. 197, 
pl. xliv, figs. 9, 10; Jukes-Browne, ‘ Cret, Rocks of Britain” vol, 1 (1900), p. 452. 
