PECTEN. 183 



raucli smaller, triangular, outer angle obtuse, marked with radial strife like tliose 

 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 

 strice as on the right valve. Ribs generally of nearly uniform size ; but in some 

 cases 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 oblicpie strige. Posterior ear small, with outer angle obtuse, and marked 

 with striae. 



Bemarlis. — 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, Jind sometimes they have a fairly distinct concentric arrangement ; in some 

 cases 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, etc. 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 Grreensand 

 (PI. XXXV, fig. 3) have ribs over the wdiole 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 Avhether these are distinct from the other forms, 

 or merely a variety. Some forms (PI. XXXV, 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 foi-nis 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, etc. (PI. 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 I am inclined to 

 regard them as constituting only a variety of P. EobinaJdinus. In some examples of the latter the 

 spines are almost, if not quite, as large as in the former. They agree better with the figures of 

 Geinitz than of Goldfuss, but the ribbing ajipears to vary considerably. Goldfuss, ' Petref. Germ.,' 

 vol. ii (1833), p. 59, pi. xciv, fig. 4; Pictet and Campiche, ' Foss. Terr. Cret. Ste. Croix' (1870), 

 p. 21-5; Geinitz, 'Das Elbthalgeb. in Sachsen ' (' Palseontographica,' vol. xx, pt. 1, 1872), p. 197, 

 pi. xliv, figs. 9, 10 ; Jukes-Browne, ' Cret. Eocks of Britain.' vol. i (1900), p. 452. 



