386 



CRETACEOUS LAMELLIBRANCHIA. 



at the actual edge, which is toothed (Plate LVII, fig. 12), was named 0. Merceyi 

 by Coquand (I860). 1 Specimens which were attached to a flat object for a long 

 period (Plate LVIII, figs. 3 — 5) were figured as 0. alsefonnis by Woodward (1833, 

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

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

 to a cylindrical object and with well-developed folds, were named 0. inaequicostata 

 by Woodward (1833, pi. vi, fig. 4), 0. sulcata by Nilsson (1827), 2 0. flabelliformis 

 by Nilsson (1833), 3 and 0. semiplana by Sowerby (1825, the larger figure) (Plate 

 LVII, fig. 7 ; Text-figs. 186, 187, 1 93). The forms like 0. alasformis and 0. Merceyi 

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

 form of 0. semiplana. 0. armata, Goldfuss, is recognised by Coquand and by 

 Midler as a form of 0. semiplana in which the ribs have become spiny. 0. 

 liclieniformis, Coquand, is a form in which the valves have a rounded outline. 



js^s, 



Fig. 193. — Oslrea semiplana, Sowerby. Zone of Belemniiella mucronata, Hartford Bridge, Norwich. Dr. Roue's 

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



0. acanthonota, Coquand, is a large form in which the ribs tend to become spiny. 

 0. arcotina, Stoliczka, is closely allied to, and may be, as maintained by Peron, 

 identical with 0. semiplana. 0. Reboudi, Coquand (— 0. plicatuloides, Coq.) and 

 0. Janus, Coquand, are regarded by Peron as forms of 0. semiplana. 



The close resemblance between the crenulations on the inner margin of the 

 dorsal part of the valves in 0. semiplana and in 0. vesicularis, and the occasional 

 presence of fine radial ribs on the right valve of 0. semiplana, suggest that these 

 two species are rather closely related. 



Remarks. — The radial folds are usually stronger in examples from the zone of 

 Belemnitella mucronata than in those from the lower zones. Occasionally the left 

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

 of the attached surface. 



1 See also Holzapfel (1889) and AVollemaim (1902). 



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

 s Henuig (1897), pi. i, fig. 16. 



