386 



CRETACEOUS la:\iellibkanchia. 



at the actual edge, ■which is toothed (Plate LVII, fig. ]2), Aras named 0. Ncrcei/i 

 l)y Coquaiul (]8(50).' Specimens which were attached to a flat object for a long 

 period (Phite LVIII, tigs, o — 5) were figured as (h niasformis by Woodward (183:3, 

 pi. vi, fig. 3) and 0. aem'rplaitn by Sowerby (1825, the smaller figure). Larger 

 forms with a relatively small portion of the anterior part of the shell attached 

 to a cylindi'ical object and wnth well-developed folds, were named 0. htxquicostata 

 by Woodward (1833, pi. vi, fig. 4), 0. sulcata by Nilsson (1827)," 0. fluheUiformis 

 l)y Nilsson (1833),^ and 0. seniiplava by Sowerby (1825, the larger figure) (Plate 

 LVII, fig. 7 ; Text-figs. 186, 187, 1 93). The forms like 0. ala'formis and 0. Merceiji 

 are common in the upper zones. 0. cnruKita, Sowerby (in Dixon, 1850), is a young 

 form of (■*. sfiiiiphuKi. 0. (iniiolu, Goldfuss, is recognised by Coquand and by 

 dialler as a form of 0. xriiiijilniKi in which the ril)S have l)ecome spiny. O. 

 lirliciiij'oi-iiiis, Co(|uand, is a form in which tlie valves have a rounded outline. 



Fici. I'jy. — Ostrca seniiplana^ Sowoi'by. Zone of Belemnllella nnufonata, Hertford Bridge, Norwich. Dr. Kowe's 

 Collection. Exterior and interior of a left valve, x i. 



0. ardiifhoitota, Coquand, is a large form in Avhieli the ribs tend to become spiny. 

 (). arculiiKi, Stoliczka, is closely allied to, and may be, as maintained by Peron, 

 identical with 0. seniipkoia. 0. BchoiiiJI, Cocpiand (= 0. pJicatiiiouIi-s, Coc\.) and 

 0. Jd.uus, Cocjuand, are regarded by Peron as forms of 0. sumiphian. 



The close resemblance between the crenulations on the inner mai-gin of the 

 dorsal part of the valves in 0. semijjlana and in 0. rt'siciih(rts, and the occasional 

 presence of fine radial ribs on the right valve of 0. seiiiipl((i/ii, suggest tliat these 

 two species are rather closely related. 



lli'inarl-s. — The radial folds are usually stronger in examples from the zone of 

 Jidimiiiltella mitcroiiata than in those from the lower zones. Occasionally the left 

 umbo is exogyriform (Plate LVII, fig. 1 i),but this depends merely on the position 

 of the attached surface. 



1 See also Holzapfel (1889) aud Wolloniatm (1902). 



2 Heuuig (1897), pi. i, fig. 18. 



3 Heuuig (1897), pi. i, tig. 16. 



