PECTEN. 193 



Bescrtption. — 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 vah'e with about twenty-four strong, rounded riljs at the margin, some 

 of which may l)e formed by bifurcation at some distance from the umbo. The ribs 

 may be all of nearly equal size, or those formed l)y bifurcation may be somewhat 

 smaller and closer together. Grooves l^etween 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 tAvo 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 libs, 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 i-ibs and concentric ridges. 



Measurements : 



(1) (2) (3) (4) (5) (6) 



Length . Wo . O'O .7.7 . G . 5-0 mm. 



Height . 11-0 . 8-75 . 7 . 6-75 . G . 475 „ 



(1, 2) B. mucrnnata zone, Norwich. 



(3, 6) „ „ Clareudon, Salisbury. 



(4) TJintacrhms zone, Devizes Eoad, Salisbury. 



(5) A. quadratus zone, East Harnham, Salisbury. 



Affinities. — This species resembles both P. acnte-pJicatits, Alth,^ and P. leopo- 

 liensis, Alth," but is distinguished from both by its more numerous ribs, the 

 narrower furrows, and concentric ridges. 



Bemarls. — This appears to be a rather rare species ; I have seen only about a 

 dozen specimens. 



7'//2)''.— From the Senonian of Chavot (Marue). 



Distrlhutiou.— TJintacrinns zone of Devizes Road, one mile west of Salisbury. 

 Marsupltes zone near Winchester. A. quadratus zone of East Harnham near 

 Salisbury, and Winchester. B. mucronata zone of Clarendon near Salisbury, and 

 of Norwich. Upper Chalk of Guildford. Chalk of Trimingham. 



1 Haidiuger's ' Naturwiss. Abhand.,' vol. iii, pt. 2 (1850), p. 248, pi. xii, fig. 34; Favre, 'Moll. 

 Foss. Craie de Lemberg' (1869), p. 148, pi. xiii, figs. 3, 4. 



' Op. cit,, p. 247, pi. xii, fig. 33; Favre, op. cit., p. 149, pi. xiii, fig. 5. 



