238 CRETACEOUS LAMELLIBRANCHIA. 



Genus — Pi.ECTOJiYA, P. de Lnriul, 1868. 

 (De Loriol and Cotteau, ' Mou. Pali'oiit. GtoI. Portlandii-n Jo I'Yonne,' p. 89.) 



Plectomya Axrir.icA, sp. iiov. Plate XXXIX, fig. In, J,. 



Description. — Sliell elongate-oval, slightly ineciuilateral, compressed; anterior 

 and posterior ends rounded j ventral margin slightly convex. Umboues broad, 

 A\ ith a sliglit posterior cm*vature. 



The part of the shell in front of a line drawn fi'om the umlio to the opposite 

 ventral margin is ornamented Avith strong concentric ribs separated by broad 

 furrows ; these ribs become nearly or quite obsolete on the posterior middle part 

 of the shell (except near the umbo), but re-appear on the postero-dorsal part. 

 The entire surface of the shell is ornamented with numerous radial rows of 

 granules. Length 49 nun ; height 27 mm. 



Affinities. — This species is similar in general character to rjertointja Agassizi 

 (d'Orbigny'), but the shell is less elongate, the ribs on the anterior part of the 

 shell cut the margin and the growth-lines obliquely, and on the jjosterior median 

 part the ribs are almost obsolete. It also shows some resemblance to 1'. EhoJiini 

 (Pictet and Ronx'-). 



liemarl-s. — Of this species I have seen only two examples, neither of which 

 shows the interior, so that I am unaljle to express an opinion as to afUnities of the 

 genus I'lecloiiujii. 



]>isfribntii)ii. — Lower Greensand (Crackers) of Atherfield." 



Genus — Anatixa, Lninarcl-, ISdU. 

 (' Philosoph. Zool," vi)l. i, p. 31SI ; ' Hist. iiat. Auiiii. saiis Vert.,' vol. v, 1818, p. 462.) 



Suh-(jenus — Ceucumya, L. Aijassb., 1S42. 

 (' Etudes crit. Moll. Foss.," Myes, p. 143.) 



1 'Pal. Franc. Terr. Cn't.,' vol. iii (1845), p. 371, pi. ci-cl.xix, figs. 1, 2; Pictet and Caiupiche, 

 'Foss. Terr. Crct. Ste. Croix' ('Mater. Pal. Suisse,' ser. 4, 1865), p. 99, pi. cvii, fig. 1. 



- ' Moll. Foss. Gres verts de Geneve' (1852), p. 410, pi. xxix, fig. 4. 



* An imperfect specimen from the Lower Greensand of Atherfield is probably an e.vample of 

 Plectomya maruUensis (d'Orbigny), op. cit., p. 376, pi. ccclxxi, figs. 3, 4 ; Pictet and Campiche, op. cit., 

 p. 101, pi. cvii, figs. 2, 3. 



