390 CRETACEOUS LAMELLIBRANCHIA. 



irregular surface. When the marginal part is free it grows more or less nearly 

 vertically upwards from the attached surface. When entirely fixed the valve may 

 be nearly symmetrical, but has usually a posterior curvature ; when attached by a 

 small area it usually grows backwards and becomes more or less sickle-shaped. 

 The ligament-pit and umbo are straight in the nearly symmetrical forms ; in the 

 curved forms they are bent or arched more or less considerably backwards, but 

 occasionally forwards. 



The right valve is slightly convex when the entire left valve is attached to a 

 flat object ; more convex when it is attached to a convex object ; nearly flat when 

 it is attached to a concave object or when attached near the umbo only ; irregular 

 and undulating when the attached surface is irregular. The outline of the valve 

 and the curvature of the ligament-pit and umbo vary according to those of the left 

 valve. The surface of the right valve sometimes shows concentric ridges, especially 

 near the umbo, and when well preserved there are numerous, small, irregular, 

 radiating ridges. The inner margin of the valve on each side of the umbo has 

 small, transverse, somewhat irregular rounded ridges. The form of the adductor 

 impression varies with the shape of the shell. 



Affinities. — Hennig (1897) has shown clearly that the differences between 

 Nilsson's 0. incurva, 0. curuirostris and 0. acutirostris are due to the mode of 

 growth, which is determined by the character and size of the attached surface ; and 

 for this species Hennig selects the name 0. incurva. 



0. triangularis, Woodward, which was named 0. trinacria by Coquand, is a 

 form in which the entire left valve is attached to a flat surface ; in the type tins 

 valve is fixed to a small portion of a large Inoceramus (Plate LIX, fig. 14). 0. 

 drepanon, Wollemann, 1 closely resembles the forms like 0. triangularis, and should 

 probably be regarded as a synonym of 0. incurva. Gryphsea glohosa, Woodward, 2 

 is included in this species by Coquand, but in the absence of the type it is difficult 

 to give a definite opinion. D'Orbigny and Peron also include 0. conirostris? 

 Golclfuss. Peron (1888, 1908) regards 0. Wegmanniana, d'Orbigny, as a form of 

 0. incurva. 0. subuncinella, Bohm, 3 appears to be closely allied to 0. incurva. 

 0. Babelaisi, Coquand, 4 from Meudon, resembles closely some forms of 0. incurva 

 (Plate LIX, figs. 12 — 14), and should probably be included in that species. 



Some specimens of 0. incurva resemble the early parts of 0. semiplana before the 

 development of the radial folds. 



Types. — Nilsson's types came from the Upper Chalk of Sweden. 0. triangularis, 



1 'Luneberger Kreide' (1902), p. 49, pi. vii, figs. 6, 7. Rogala, 'Bull. Internal;. Acad. Sci. 

 Cracovie ' (1909), 2, p. 691, pi. xxviii, fig. 16. 



2 'Geol. Norfolk ' (1833), p. 52, pi. vi, fig. 8 (from Marham). 



3 ' Palseoutograpliioa,' vol. xxxviii (1891), p. 93, pi. iv, figs. 9, 10. 

 * ' Mon. Ostiea, Terr. CreV (1869), p. 66, pi. xxxvii, figs. 26, 27. 



