PECTEN. 193 
Description.—Shell small, nearly equilateral, nearly circular, length equal to, or 
slightly greater than the height. Antero- and postero-dorsal margins straightened. 
Convexity small. Apical angle about 106°. Ears moderately large, unequal. 
Right valve with about twenty-four strong, rounded ribs at the margin, some 
of which may be formed by bifurcation at some distance from the umbo. The ribs 
may be all of nearly equal size, or those formed by bifurcation may be somewhat 
smaller and closer together. Grooves between the ribs are deep, but rather narrower 
than the ribs. Both grooves and ribs are crossed by concentric lamellar ridges, 
which are especially distinct near the umbo, and may be absent or indistinct 
ventrally; these ridges are continuous, and placed at regular intervals. Anterior 
ear long, with a deep sinus, and two or three serrate ribs radiating from its apex 
and crossed by fine concentric ridges. Posterior ear smaller, triangular, nearly 
smooth. 
Left valve with similar ornamentation, but rather narrower ribs, some of which 
do not reach the neighbourhood of the umbo. Anterior ear rather long, with three 
or four well-marked radial ribs crossed by concentric ridges. Posterior ear 
smaller, with rather less distinct radial ribs and concentric ridges. 
Measurements : 
(1) (2) (3) (4) (5) (6) 
Henoth) >. lis". 9:0 @ ( 692 0.0) mm: 
ei hteee LICOW Onion. fs TORO): ge Oamemmeiommes 
(1,2) B. mucronata zone, Norwich. 
(3, 6) am re Clarendon, Salisbury. 
(4) Uintacrinus zone, Devizes Road, Salisbury. 
(5) A. quadratus zone, East Harnham, Salisbury. 
Affinities—This species resembles both P. acute-plicatus, Alth,’ and P. leopo- 
liensis, Alth,? but is distinguished from both by its more numerous ribs, the 
narrower furrows, and concentric ridges. 
Remarks.—This appears to be a rather rare species; I have seen only about a 
dozen specimens. 
Type.—F rom the Senonian of Chayot (Marne). 
Distribution.—Uintacrinus zone of Devizes Road, one mile west of Salisbury. 
Marsupites zone near Winchester. A. quadratus zone of East Harnham near 
Salisbury, and Winchester. 2. mucronata zone of Clarendon near Salisbury, and 
of Norwich. Upper Chalk of Guildford. Chalk of Trimingham. 
1 Haidinger’s ‘Naturwiss. Abhand.,’ vol. iii, pt. 2 (1850), p. 248, pl. xii, fig. 34; Favre, ‘ Moll. 
Foss. Craie de Lemberg’ (1869), p. 148, pl. xiii, figs. 3, 4. 
2 Op. cit., p. 247, pl. xii, fig. 33; Favre, op. cit., p. 149, pl. xiii, fig. 5. 
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