12G CRETACEOUS LAMELLIBRANCHIA. 



sometimes less inflation than the left ; the attached part bears concentric 

 toothed lamellae ; the free surface has numerous regular, nearly equal radial ribs, 

 separated by narrow grooves ; in some cases the ribs carry short spines placed 

 rather irregularly, and sometimes slender transverse ribs are seen. 



Left valve usually regularly convex, with numerous slightly unequal ribs, which 

 are occasionally a little wavy. The ribs are separated by grooves, generally of 

 greater, but sometimes of the same width. The spines on the ribs are hollow, 

 usually short, sometimes perpendicular to the surface, sometimes sloping ventrally ; 

 they may be developed rather irregularly, or larger spines may be borne on slightly 

 stronger ribs at regular intervals ; between these ribs are three or two slightly 

 smaller ribs, with sometimes smaller spines. Faintly marked transverse ribs occur 

 in the grooves, and sometimes extend on to the ribs. Ears smooth, except for 

 growth- lines. 



Measurements of left valve : 

















(1) (2) 



(3) 



(4) 



(5) 



(6) 



(7) 



(8) 



(9) 



Length . 20 22'5 



23 



23 



26 



17 



22 



27 



24 mm, 



Height . 23 25 



25 



26-5 



30 



19-5 



26 



30 



27 „ 



(1 — 5) zone of A. quadratus. East Harnham. 

 (6 — 8) „ ,, West Harnham. 



(9) Chalk of Triuiiugham. 



Affinities. — 8. Onialii, d'Archiac,^ appears to be a more oblique form with 

 fewer ribs. The form referred by d'Orbigny ^ to 8. hystrix is larger than 

 8. Dntempleanus, and has the spines confined to the neighbourhood of the umbo. 

 Specimens from the Cenomanian of Devon and Orbiquet (Normand}-) agree with 

 the Turonian and Senonian examples, except that the ribs are a little more widely 

 separated. Similar Cenomanian forms from St. Fargeau (Yonne) and St. Sauveur 

 (Yonne) have also been referred to 8. Dutempleanus by d'Orbigny and by Peron. 



8. flinbriatiis, Groldfuss,^ is very similar to 8. DiUempleanus ; judging from the 

 figures alone, the only difference appears to l)e in the fewer spines on the 

 ribs. 



A specimen, described and figured by Sowerby* as Lima? spinosa, is perhaps 

 an example of 8. Dutempleanus. I have not been able to find the original, and the 

 locality is not stated. 



Uentarlcs. — In the majority of specimens seen the spines have been more or less 

 completely broken in the process of clearing away the matrix. Examples from 



1 ' Mom. Soc. geol. de France,' vol. ii (1847), p. 312, pi. xv, fig. 11. 



2 ' Pal. Franc. Terr. Crct.,' vol. iii (1847), pi. ccccliv, figs. 1—9. 



^ ' Fetref. Germ.,' vol. ii (1836), p. 97, pi. cvi, fig. 2. A specimen was referred to this species by 

 Dixon ('G-eol. Sussex,' p. 356, pi. xxviii, fig. 34). 



^ F. Dixon, ' Geol. Sussex' (1850), p. 347, pi. xxviii, fig. 33; p. 382 of ed. 2. 



