Bivalves from the white chalk (Maastrichtian) of Denmark, II: Arcoida CLAUS HEINBERG Heinberg, C: Bivalves from the white chalk (Maastrichtian) of Denmark, II: Arcoida. Bull. geol. Soc. DGF Denmark, vol. 27, pp. 105-116, Copenhagen, March 23rd, 1979. Eight new species of Arcoida (Bivalvia) are described, the genus belonging to the families Limopsidae, Arcidae, Noetidae and Parallelodontidae. Two species (Barbatia) are redescribed and revised. Pseudo- grammatodon Aikell, 1930, is reinstated. Claus Heinberg, Institut for historisk Geologi og Palæontologi, Øster Voldgade 10, DK-1350 København K, Denmark. July 4th, 1978. The present paper is the continuation of a pro­ Type locality: Stevns Klint, north of Kulsti. ceeding one on Limopsidae (Heinberg 1976). Type stratum: The top hardground in the Upper The first paper contained information on biome- Maarstrichtian white chalk. tric parameters, collecting procedures, localities and lithostratigraphic position of the hardground Diagnosis: An equilateral, smooth shelled Li­ which has yielded the material. mopsis, flat lenticular in shape, subcircular in outline; umbo small; shell margins moulding each Systematic descriptions other, the right margin having a commarginal ridge which fits into the commarginal furrow of Family: Limposidae Dall, 1895 the left margin; equal sized adductor scars with Genus: Limopsis Sassi, 1827 myophoric flanges; area circumscribed by pallial Type species: Area aurita Brocchi, 1814 line bearing radiating striae. Limopsis maggae n. sp. Description: The shell is flat lenticular having a Fig. 1 subcircular outline. It is nearly equilateral, the Holotype: MGUH 14400. Height 5.4 mm, length anterodorsal shell margin being straighter than ca 6.5 mm the postero-dorsal one. The orthogyrate umbo is Fig. 1. Limopsis maggae n.sp. Al: Inside of a right valve, latex cast, holotype, MGUH 14400. x8. Bl, B2: Outer surface of a left valve seen in side and anterior views, MGUH 14401. x8. 106 Heinberg: Maastrichtian bivalves very small. The umbonal part of the shell is only Remarks: The peculiar shell margin is distinctive slightly elevated, causing the dorsal flanks to be for the species. only a little steeper than the rest of the shell. The hinge line is straight and the rest of the shell mar­ Material: 38 specimens from Stevns Klint, gin forms a gentle curve which meets the hinge including five from north of Kulsti, seven from line at obtuse angles. The cardinal area is a flat north of Eskesti chalk pit, eight from north of symmetrical triangle with faint striae which are Holtug chalk pit, eleven from Rødvig and seven parallel to the hinge line. There is a triangular from other parts of the cliff. resilifer below umbo. The resilifer is divided into a median and two lateral sections, reflecting the Family Arcidae Lamarck, 1809 alivincular type of ligament. The dentition is divi­ Subfamily Arcinae Lamarck, 1809 ded into two subsymmetrical series by an eden­ Genus Area Linné, 1758 tulous gap. The anterior series consists of up to Type species: Area noae Schmidt, 1818 seven, the posterior of up to eight teeth. All the hinge teeth radiate with respect to an area ventral of umbo. The shell surface is smooth, with regu­ Area (Area) ernae n.sp. larly spaced growth lines. The smooth margins of Fig. 2 the two valves are moulds of each other, the right Holotype: MGUH 14402. Height 4.6 mm, length valve having a convex, the left a concave profile. 9.1 mm, width 6.8 mm. The inner surface of the shell is smooth with radi­ Type locality: Stevns Klint, north of Eskesti chalk ating striae characterising the area circumscribed pit. by the pallial line. The two adductor scars are Type stratum: The top hardground in the Upper often clearly marked. They are of almost equal Maastrichtian white chalk. size and bordered by distinct myophoric flanges, the posterior of which runs into the umbonal ca­ Diagnosis: A relatively small, broad Area (Area) vity. having a high, flat cardinal area; outline rhom- boidal; distinct carina bordering plane, heavily Maximum dimensions: Height 7.7. mm, length ribbed corselet; anterior and posterior shell mar­ 8.5 mm, width 3.4 mm. gins bearing distinct crenulae. A2 Fig. 2. Area (Area) ernae n.sp. Al, A2, A3, A4: Right valve seen in lateral view, from the inside, in posterior and dorsal views, latex cast, holotype, MGUH 14402. x7.5. B: Outer surfa­ ce of a left valve, small individual, latex cast, MG UH 14403. xl4. Bulletin of the Geological Society of Denmark, vol. 27 1978 107 Descriptions: The shell is rhomboidal in outline. on the ribs of the corselet, knobs on rest of the It is equivalve, prosocline, the praeumbonal part shell. The inner surface of the shell is smooth, as of the shell constituting ca 32% of the total shell is the ventral margin. The posterior and the ante­ length. It is heavily in flated, the width of a single rior margins are dominated by large crenulae valve being ca 0.7 times the height. The hinge line corresponding to the radiating ribs. is long and straight, the anterior and posterior shell margins are subparallel trending in an an- Maximum dimensions: Height ca 5 mm, length tero-dorsal/postero-ventral direction. The ante­ 10.5 mm, width 7.8 mm. rior margin is gently curved, continuing with no major change in curvature into the long sinus, Material: 20 specimens from Stevns Klint, which occupies a large part of the ventral margin. including eight from north of Holtug chalk pit, The relatively straight posterior margin forms a seven from north of Eskesti chalk pit, one from distinct angle with the ventral. In smaller individ­ Kulsti, two from Rødvig, and two from other uals this angle is acute owing to a characteristic parts of the cliff. The Lower Maastrichtian chalk posterior projection of the postero-ventral shell of Møns Klint has yielded three specimens. corner. In larger specimens this projection disap­ pears and the angle becomes obtuse. The umbo is Genus Barbatia Gray, 1842 small, hardly projecting above the cardinal area. Type species: Area barbata Linné, 1758 The cardinal area is a flat triangle of considerable height (the height being ca 40% of the base). The Barbatia (Barbatia) forchammeri (Lundgren, cardinal area is striated with straight growth lines 1888) converging anteriorly. Ligament grooves descend Fig. 3 from the umbo and bend towards the anterior as 1867, Area striata Lundgren, p. 27. PL 1, Fig. 8 the hinge is approached. The hinge consists of a 1888, Area forchammeri nov.nom. Lundgren, p. continuous series of rather uniform small teeth 229 which radiate with respect to a point ventral of 1899, Barbatia tenuidentata Henning, p. 16, PL 1, the umbo. On the shell surface a distinct carina Fig. 9-11 separates the plane corselet from the rest of the 1902, Barbatia tenuidentata Henning, Ravn, p. shell. The sculpture is dominated by round-cre­ 54, PL 3 Fig. 9 sted, radiating ribs which are most pronounced 1933, Barbatia faxensis Ravn, p. 13, PL 1, Fig. on the corselet. At a distance of ca 5 mm from 2a~b umbo, the ribs occur at a density of three per mm on the flank, two per mm on the corselet. Growth Due to taxonomic confusion of the Dani- lines crossing the radiating ribs are developed to a an/Maastrichtian barbatias, the species is rede- varying degree. Distinct growth lines form scales scribed on the basis of a specimen from the top Al A2 ----- - --"V B iKi Fig. 3. Barbatia (B.) forchammeri (Lundgren). Al, A2: Outer and inner surfaces of a right valve, latex cast of MGUH 14404. x4.5. B: Outer surface of a left valve, latex cast, MGUH 14405. x4.5. 108 Heinberg: Maastrichtian bivalves hardground in the Upper Maastrichtian white Discussion: There is close morphological re­ chalk at Stevns Klint. Material from the Danian semblance among the Maastrichtian Barbatia deposits at Fakse and Annetorp has been exami­ described above, and the two Danian species B. ned in the absence of the original type material, faxensis Ravn, 1933 and B. forchammeri (Lund­ which is lost. gren, 1888). While the Maastrichtian specimen here described has a small antero-dorsal angle (ca Diagnosis: An elongate, ovate Barbatia, proso- 100°), there is a tendency for large individuals gyrate, having subparallel dorsal and ventral from the Danian to have a larger antero-dorsal margins; dominant radiating and subdominant angle (ca 130 °). However, the 100° angle is well commarginal ribbing, crosspoints of ribs devel­ within the range of variation of the Danian mate­ oped as tubercules or scales; posterior margin rial. commonly undulating. It thus seems that, among large specimens of this species group, Danian individuals tend to Description (based on MGUH 14404): The shell have a more projecting anterior part than is equivalve, inequilateral with a smooth rounded Maastrichtian ones, smaller individuals being al­ ovate outline. The hinge line is straight, rest of most identical. On the basis of the variation range the shell margin forming a gentle curve with the in the present material this difference is not re­ maximum curvature occurring posteriorly. The garded as sufficient for the creation of a separate antero-dorsal shell angle is about 100°, the Maastrichtian species, and, further, Ravn's postero-dorsal about 135°. The prosogyrate um­ (1933) Barbatia faxensis is regarded as a junior bo is prosocline, situated about 30% of the total synonym. Thus all specimens here dealt with are length behind the anteriormost point of the shell. regarded as the same species. In any case, it se­ The shell is moderately inflated. There is indica­ ems obvious that we are dealing with one tion of a sulcus connecting the umbo with the evolutionary lineage. ventral margin. No carina is present.